A PDF is usually big because of the images inside it, not the text. Scanned pages and photos are stored as pictures, and a single page scanned in color at a high resolution can carry millions of pixels. Compressing a PDF mostly means re-saving those pictures at a lower resolution and JPEG quality. Typed text barely takes any space, so a PDF exported from Word shrinks only a little, while a 20-page scan can shrink a lot. You can do this in your browser with Compress PDF, without sending the document to anyone's server.
Why some PDFs are huge and others are tiny
A PDF can hold two very different kinds of content:
- Text and vector graphics. A typed page is stored as instructions: which characters, in which font, at which position, plus lines and shapes for tables and charts. That is compact, and it stays sharp at any zoom.
- Images. A scanned page is one big photo of the paper. Photos pasted into a report are images too. Their size depends on how many pixels they have and how they are compressed.
PDF measures pages in units of 1/72 inch, and an A4 page is 8.27 × 11.69 inches. Scanned at 300 DPI, that page becomes about 2480 × 3508 pixels, roughly 8.7 million pixels. Stored as uncompressed color at 3 bytes per pixel, that would be about 26 MB for one page. Scanners and scanning apps compress this, but a multi-page color scan still adds up quickly, which is why a short stack of paperwork can produce a PDF too big to email.
What compression changes, and what it doesn't
Compress PDF does two things. It re-saves photos and scanned images as JPEG at a lower resolution and quality. JPEG is a lossy format: it discards fine detail to make files smaller, which suits photos well. And it compacts the file's structure, packing the PDF's internal objects into compressed object streams and recompressing other streams, the same techniques the open-source tool qpdf documents.
It does not touch text or vector graphics. Typed text stays real text: sharp, selectable and searchable. The exception is text that is part of a scan, because there the text is pixels in an image, so it gets slightly softer as the image is scaled down.
There are three levels. The resolution is the longest side of each image; the DPI column shows what that means for a scan that fills an A4 page.
| Level | Image long side | JPEG quality | Full-page A4 scan |
|---|---|---|---|
| Strong | about 1240 px | 55 | about 106 DPI |
| Balanced (default) | about 1600 px | 70 | about 137 DPI |
| Light | about 2400 px | 82 | about 205 DPI |
Compared with that 300 DPI scan, Balanced keeps about a fifth of the pixels and Strong about an eighth, before JPEG quality saves even more.
Picking a level for an email limit
Google's help page says the attachment limit for personal Gmail accounts is 25 MB, and that if the total attachment size is greater than the limit, Gmail removes the attachment and adds it as a Google Drive link instead. For work and school accounts, the administrator sets the limits. Other email services and upload forms have their own limits, so check the number that applies to you.
- Open the PDF in Compress PDF and keep Balanced selected.
- Compare the size before and after. If it's under the limit with some room to spare, zoom in on small print to check it's still readable.
- If it's still too big, try Strong. If small text in a scan looks blurry at Balanced, try Light and see whether that still fits.
- If even Strong isn't enough, split the document with Split PDF and send the parts in separate emails.
If you're building the PDF from phone photos, it's often easier to shrink the photos first with Compress image and then combine them with JPG to PDF.
When compression won't help
- Mostly-text PDFs. Documents saved from Word or a similar program have little image data to shrink.
- Already compressed files. If the images are already small, there isn't much left to remove. When the result wouldn't be smaller, Compress PDF gives you the original back with a note rather than a bigger file.
- Unusual image formats. Images stored in CMYK, 16-bit, JPEG 2000 or indexed color are left as they are.
- Password-protected files. A PDF that needs a password to open has to be unlocked first with Unlock PDF, which requires the password. PDFs that only restrict copying or printing can be compressed directly.
Why compress in the browser
The documents people most often need to shrink are tax forms, contracts, ID scans and medical records. Many online compressors upload the file, process it on their server and send it back. Compress PDF runs inside your browser tab instead, so the document stays on your device and there is no upload to wait for. The practical limit is your device's memory, so very large PDFs are best handled on a computer.