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When an email attachment is too large to send, compression is usually the fastest and most straightforward solution. Different file types respond very differently to compression — some can be reduced by 90%, while others barely shrink at all. This guide covers ZIP compression on both Windows and Mac, dedicated PDF compression, image optimization strategies, expected compression ratios by file type, and how to protect sensitive compressed archives with a password.
Windows has built-in ZIP support that requires no additional software:
Note that Windows’ built-in ZIP uses the Deflate algorithm, which is widely compatible but not the most efficient available. For significantly better compression ratios, consider installing 7-Zip (free and open-source) or WinRAR, which support more modern algorithms. In 7-Zip, you can choose between ZIP, 7z, and other formats — the 7z format with LZMA2 compression generally outperforms ZIP by a substantial margin for text-heavy files.
macOS also includes built-in compression, accessible through the Finder:
For more control on Mac, you can use the Terminal with the zip command, specifying compression levels from 0 (store only, no compression) to 9 (maximum compression). Third-party apps like Keka or The Unarchiver offer additional format support and a friendlier interface for advanced options.
This is the most important thing to understand about compression: not all files compress equally. Many modern file formats are already compressed internally, which limits how much further you can reduce them with ZIP.
| File Type | ZIP Compression Savings | Reason |
|---|---|---|
| Plain text (.txt, .csv) | 60–90% | Uncompressed, highly repetitive data |
| Word documents (.docx) | 10–30% | .docx is already a ZIP container |
| PDF documents | 5–20% | PDFs have internal compression |
| JPEG images | 0–5% | Already heavily compressed |
| PNG images | 5–15% | Uses lossless compression internally |
| MP4 video | 0–2% | Already heavily compressed |
| BMP / TIFF images | 40–70% | Uncompressed raw formats |
| Excel spreadsheets (.xlsx) | 10–25% | .xlsx is already a ZIP container |
The key takeaway: if you’re trying to compress a JPEG photo or an MP4 video, ZIP will barely help. For uncompressed formats like BMP, TIFF, or plain text, ZIP compression is highly effective. For PDFs and Office documents, use format-specific compression tools instead of (or in addition to) ZIP.
PDF is the standard format for business documents — contracts, invoices, reports, proposals — and it’s worth knowing how to compress it effectively. Bloated PDFs typically result from high-resolution embedded images, unoptimized fonts, or metadata.
Online PDF compression tools: Upload your PDF to a tool like iLovePDF, Smallpdf, or PDF Compressor, choose a compression level (usually labeled “light,” “recommended,” or “high”), and download the result. The whole process takes about 30 seconds.
Expected results by PDF type:
Compressing PDF with Adobe Acrobat (paid): File → Save As Other → Reduced Size PDF. This gives you granular control — you can specify target image DPI (72 DPI for screen, 150 DPI for print), manage embedded fonts, and remove hidden layers or metadata.
Free compression on macOS using Preview: Open the PDF in Preview, go to File → Export as PDF, and in the Quartz Filter dropdown, select “Reduce File Size.” This applies Apple’s built-in compression filter, which is free but less precise than dedicated tools — it works well for simple documents but may over-compress image-heavy files.
If your attachment is primarily images, you have more options than just ZIP:
Resize the resolution (most effective): A photo from a modern smartphone or DSLR camera can be 10–25 MB in its original resolution (4000 × 3000 pixels or higher). For email, there’s rarely any reason to send at full resolution — screens display at 72–96 DPI. Resizing to 1920 × 1440 or even 1280 × 960 can reduce the file size by 75% or more with no visible quality difference on screen.
Convert to a more efficient format:
Adjust JPEG quality settings: JPEG uses lossy compression, meaning lower quality settings produce smaller files at the cost of some detail. A quality setting of 80 (on a 0–100 scale) is generally the sweet spot — it produces files 50–70% smaller than quality 100 while the difference is virtually invisible to the naked eye for most types of content.
If your compressed attachment contains sensitive documents — contracts, financial statements, personal information — you can encrypt the archive with a password to prevent unauthorized access:
Using 7-Zip on Windows (free):
On Mac using Keka or the Terminal: Keka supports AES-256 encrypted ZIP and 7z archives through its graphical interface. In Terminal, you can use zip -e archive.zip file.pdf to create a password-protected ZIP.
Critical security note: Never send the password in the same email as the encrypted archive — doing so defeats the purpose entirely. Share the password through a different channel: a phone call, SMS, or a separate messaging app. This way, even if someone intercepts the email, they cannot open the archive without the separately communicated password.
After compressing, take a moment to verify the result before attaching it:
Choosing the right compression approach for your file type makes the difference between a quick fix and a frustrating exercise in shrinking something that was never going to get significantly smaller. Match the method to the material, and you’ll rarely have trouble getting files through.
If the file is confidential, the safest route is the one where it never leaves your computer. PDF Compress runs entirely in the browser: the file is read from your disk, processed in memory, and written back — nothing is sent to a server.
Do it in your browser — the file never leaves your device: PDF Compress