How to Reduce a PDF's File Size (Without Ruining It)
A PDF that won't attach to an email or upload to a portal is one of the most common small annoyances there is. The fix depends on why the file is big in the first place.
Why PDFs get big
Text is tiny. A page of pure text in a PDF is usually a few kilobytes. When a PDF weighs tens of megabytes, the cause is almost always one of these:
- Images. Scanned pages and embedded photos are the overwhelming cause. A scan is really a photograph of a page, so a 20-page scan at high resolution can easily exceed 30 MB.
- Embedded fonts. Some documents carry several complete font files rather than only the characters used.
- Leftover data. Editing histories, thumbnails, unused objects and form data can add weight.
- Very high image resolution. A photo at 600 dpi looks the same on screen as at 150 dpi but contains sixteen times the pixels.
What compression actually does
Because images dominate, most PDF compression works by re-encoding those images: lowering their resolution, applying stronger JPEG compression, or both. Text and vector graphics are left largely alone. That is why compressing a text-only PDF barely helps, while compressing a scan can cut it by 70–90%.
Some tools, including our Compress PDF, take a blunter route: they re-render each whole page as an image at a chosen resolution. That is very effective on scans, but any selectable text becomes part of the picture, so it is the wrong choice for a text-based document you need to search or copy from.
The trade-off is image quality. Compress gently and the document looks identical on screen; compress hard and small print and photos begin to look soft or blotchy. A good rule is to pick the strongest setting that still leaves the smallest text comfortably readable at 100% zoom.
Know your size target
- Email: Gmail allows 25 MB per message and Outlook is usually similar, but many company servers are stricter. Under 10 MB is a safe aim.
- Job and visa portals: often 2–5 MB per file. Check the stated limit before you start so you don't compress more than you need.
- Web pages: for a PDF people will open in the browser, a few megabytes keeps it fast on mobile.
A step-by-step approach
- Check the size and page count. Ten MB across ten pages means images are the issue.
- Compress at a moderate level first and look at the result. Zoom to 100% and read the smallest text.
- Still too big? Step up one level. Stop as soon as you are under the limit.
- Split if needed. Some limits are per file. Separating a long document into two parts can be simpler than crushing every page. See how to merge and split PDFs.
- Keep the original. Send the compressed copy, archive the full-quality one.
Better scans from the start
If you make the scans yourself, the cheapest saving happens before the PDF exists. Scan text documents at 150–200 dpi in black and white or greyscale rather than 600 dpi in colour. Most phone scanning apps offer a "document" mode that does this. A 10-page contract scanned that way is often under 1 MB.
Why privacy matters here
PDFs are frequently contracts, IDs, tax forms and medical letters. Many "compress PDF online" services upload your file to their servers. Our compressor runs in your browser, so the document never leaves your device — the same as if you did it in a desktop app, without installing one.
Frequently asked questions
Why didn't my PDF get smaller?
If it is mostly text and vector graphics, there is little to squeeze. Files that are already optimised, or that contain only a few low-resolution images, can even end up slightly larger.
Will compression remove the text or make it unsearchable?
It depends on the tool. Compressors that only re-encode embedded images keep selectable, searchable text. Our Compress PDF re-renders every page as an image, so the text is no longer selectable or searchable afterwards. A scanned page has no real text either way unless it was run through OCR.
Is there a size limit for the tool?
The limit is your browser's memory, since the file is processed on your device. Very large files, several hundred megabytes, may struggle on a phone.
Can I compress a password-protected PDF?
You will need to remove the restriction or open it with the password first.