File compression is one of the most common needs in everyday work. An email attachment exceeding 25MB cannot be sent, a PDF file is too large to upload to a system, a set of high-resolution images fills up your hard drive—all these scenarios point to the same problem: the file size needs to be reduced.
This guide starts from the fundamental principles of compression and systematically covers five core topics: PDF compression, image compression, video compression, OFD compression, and enterprise file processing compliance, helping you understand the compression logic for different file types and choose the most suitable solution.
1. Fundamental Principles of File Compression
The essence of file compression is expressing the same information with less data. Based on whether information is lost, compression is divided into two main categories:
1.1 Lossless Compression
Lossless compression reduces file size while ensuring that the decompressed data is identical to the original. Its principle is eliminating redundancy in the data—for example, a sequence of repeated data "AAAAABBBCC" can be encoded as "5A3B2C", immediately halving the storage space.
Typical applications: ZIP/7z archive compression, PNG images, FLAC audio. Lossless compression is suitable for text, code, spreadsheets, and other scenarios where no information can be lost.
1.2 Lossy Compression
Lossy compression achieves smaller file sizes by discarding information that the human eye or ear is less sensitive to. For example, a 4000×3000 photo only needs 1920×1080 pixels for screen display, so the extra pixel information can be safely discarded.
Typical applications: JPEG images, MP3 audio, MP4/H.264 video. Lossy compression is suitable for images, video, audio, and other scenarios where some quality loss is acceptable.
Understanding the difference between these two compression methods is the foundation for choosing compression tools and parameters. Most document compression (such as PDF and Word) uses both methods simultaneously—text is compressed losslessly, while embedded images and video are compressed lossily.
2. Archive Compression vs. Content Compression: A Key Distinction
Many people confuse two entirely different compression needs:
| Dimension | Archive Compression (7-Zip/WinRAR) | Content Compression (SmartSlim) |
|---|---|---|
| What it does | Packages files into .zip/.7z/.rar | Directly reduces the file's own size |
| Compression target | Collections of any files | PDF, images, video, Office documents |
| Compression method | Lossless (eliminates redundancy) | Lossy + lossless hybrid (optimizes content) |
| Usage | Must decompress before use | Open and use directly after compression |
| Typical scenario | Packaging multiple files for transfer | Reducing individual file size |
Simply put: if your problem is "the file is too large to send by email," you need content compression, not archive compression—because the file extracted from the .zip by the recipient will still be the same size. For a detailed explanation of this distinction, see our article on PDF Compression Principles & Methods.
3. PDF Compression
PDF is the most common file type that needs compression in office scenarios. Contracts, reports, courseware, electronic invoices—a large number of documents circulate in PDF format, and embedded high-resolution images, scanned documents, and vector graphics often cause size bloat.
PDF compression is primarily achieved through the following methods:
- Image resampling: Downscaling embedded high-resolution images to appropriate sizes (e.g., 300dpi → 150dpi)
- Image format conversion: Converting uncompressed BMP/TIFF to JPEG/WebP
- Font subsetting: Keeping only the characters actually used in the document, discarding unused glyphs
- Removing redundant objects: Cleaning up duplicate stream objects and unreferenced resources within the PDF
- Linearization optimization: Reorganizing the PDF's internal structure to improve loading speed
An 80MB scanned PDF, with proper image resampling and format conversion, can typically be compressed to 10-15MB while keeping the text clearly readable. The key is choosing the right compression level—150dpi is sufficient for email scenarios, while 300dpi is recommended for printing.
For more practical details on PDF compression, read PDF Compression Principles & Methods: How to Effectively Reduce PDF Size.
4. Image Compression
Images are the primary cause of file size bloat. A single smartphone photo can be 5-10MB, and a Word document with a dozen embedded images can easily exceed 50MB. Image compression is one of the most effective ways to reduce document size.
Different image formats have very different compression characteristics:
| Format | Compression Type | Best Use Case | Characteristics |
|---|---|---|---|
| JPEG | Lossy | Photos, gradient images | High compression ratio, no transparency support |
| PNG | Lossless | Icons, screenshots, transparent images | Larger file size, supports alpha channel |
| WebP | Lossy/Lossless | Web images | 25-35% smaller than JPEG, good compatibility |
| TIFF | Lossless/Uncompressed | Printing, professional photography | Largest file size, highest quality |
| BMP | Uncompressed | Rarely used | Enormous file size, not recommended |
The core parameters for image compression are resolution and quality factor. Reducing a 4000×3000 photo to 1920×1080 can reduce the size by over 75%; lowering JPEG quality from 95 to 75 is virtually indistinguishable to the naked eye but reduces the size by 50%.
For detailed selection strategies and compression parameter tuning for different image formats, see Image Compression Guide: JPG/PNG/WebP Format Comparison & Selection.
5. Video Compression
Video has the largest file size among all file types. A 1-minute 4K video can exceed 400MB, and a 1080p screen recording can also be over 100MB. Video compression is key to reducing storage and transmission costs.
The core of video compression is choosing the right codec:
- H.264 (AVC): Best compatibility, playable on virtually all devices. Suitable for general use.
- H.265 (HEVC): Saves 40-50% in size compared to H.264, but some older devices do not support it.
- VP9: Developed by Google, compression efficiency close to H.265, primarily used on web platforms.
- AV1: The latest codec with the highest compression efficiency, but slower encoding and hardware support is still being adopted.
In addition to codec selection, video compression also involves bitrate control (CRF/CBR/VBR), resolution adjustment, and frame rate adjustment. Reducing 4K to 1080p, or frame rate from 60fps to 30fps, can significantly reduce file size.
For users who need to send video via email, we recommend H.264 encoding + 720p resolution + 30fps + CRF 28. A 1-minute video can typically be kept under 10MB.
For a more detailed video compression parameter guide, see Video Compression Guide: How to Reduce Size While Maintaining Quality.
6. OFD Compression
OFD (Open Fixed-layout Document) is a fixed-layout document format defined by the Chinese national standard GB/T 33190-2016, widely used in electronic document systems of government agencies and public institutions. Similar to PDF, OFD files can also bloat due to embedded large amounts of images.
The principle of OFD compression is similar to PDF compression—optimizing embedded images, subsetting fonts, and cleaning up redundant objects. However, since OFD is less commonly used outside of government contexts, tools that support OFD compression are very scarce, which is why SmartSlim was among the first to support OFD compression.
For a detailed comparison of OFD and PDF and compression methods, read OFD vs PDF: Differences and Compression Methods.
7. Local vs. Online Compression: Security Considerations
Compression tools come in two modes: local compression and online compression:
Online compression (such as Smallpdf, iLovePDF) requires uploading files to a server for processing. The advantage is no software installation needed; the disadvantage is that your file data leaves your device. For sensitive files like contracts, ID scans, and financial statements, uploading to third-party servers poses data leakage risks.
Local compression (such as SmartSlim) completes the entire compression process on your computer—files never leave the device and no network connection is required. For individual users concerned about data privacy and enterprise users subject to compliance requirements, local compression is the safer choice.
With the implementation of the Data Security Law and the Personal Information Protection Law, enterprises must consider data export and privacy compliance when choosing file processing tools. See Enterprise File Processing Compliance Guide Under the Data Security Law for details.
8. How to Choose a Compression Tool
When choosing a compression tool, we recommend evaluating the following dimensions:
| Dimension | Key Questions |
|---|---|
| Compression capability | Can it handle PDF, images, video, Office documents, and other formats? |
| Compression effectiveness | What is the compression ratio at equivalent quality? Does it support multiple compression levels? |
| Data security | Do files need to be uploaded? Local processing or cloud processing? |
| Batch processing | Can it compress multiple files simultaneously? Does it support queue management? |
| Platform support | Does it support Windows, macOS, Linux? Does it support Apple Silicon? |
| Resource usage | Does compression affect other programs? Is CPU/memory usage controllable? |
| Cost | What are the free version limitations? Is the paid version reasonably priced? |
For users who only occasionally compress one or two files, free online tools may suffice; but for users who frequently process large numbers of files or have data security requirements, a local compression tool is the more appropriate choice.
9. Frequently Asked Questions (FAQ)
Will compression reduce image quality?
It depends on the compression method. Lossless compression does not reduce quality, but lossy compression sacrifices some detail for a smaller file size. Choosing the right compression level allows you to balance size and quality. Generally, compressing PDF to 150dpi is perfectly adequate for screen reading, and the difference is virtually imperceptible to the naked eye.
What is the difference between local and online compression?
Local compression is performed on your computer, so files are never uploaded to the network, ensuring high privacy and security without depending on network speed. Online compression requires uploading files to a server, which poses data leakage risks but requires no software installation. For sensitive files like contracts and financial statements, local compression is recommended.
How do I compress a PDF that is too large?
PDF compression is primarily achieved by reducing embedded image resolution, resampling images, subsetting fonts, and removing redundant objects. Professional compression tools let you choose compression levels to balance size and clarity. For email scenarios, simply select the "email" mode.
What is the difference between 7-Zip and SmartSlim?
7-Zip is an archiving tool that packages files into .7z/.zip archives, restoring the original files upon extraction with no size change. SmartSlim is a content compression tool that directly reduces the size of PDF, image, and video files. The compressed files can be opened and used directly without decompression.
Conclusion
File compression is not a complex technology, but choosing the right method and tool can make all the difference. The core principle is: clarify your compression goal (email/web publishing/storage archiving), choose the appropriate compression method (lossy/lossless), and use a secure and reliable tool (local compression).
If you are looking for a multi-format, locally processed, cross-platform compression tool, try SmartSlim—supporting full-format compression for PDF, OFD, Word, Excel, PPT, images, video, and audio, with files never uploaded and the entire process completed locally.
Related Reading:
- PDF Compression Principles & Methods: How to Effectively Reduce PDF Size
- Image Compression Guide: JPG/PNG/WebP Format Comparison & Selection
- Video Compression Guide: How to Reduce Size While Maintaining Quality
- OFD vs PDF: Differences and Compression Methods
- Enterprise File Processing Compliance Guide Under the Data Security Law