Two kinds of PDF compression
PDF files get large for different reasons, so this compress PDF tool offers two very different methods.
Lossless – keep text selectable (smaller gains). The PDF is rewritten by qpdf, a respected open-source PDF engine: internal streams are recompressed at the highest level, objects are packed into compressed object streams, and resources that nothing uses any more are removed. Nothing you can see changes – text stays selectable and searchable, links and form fields remain, images keep their quality. Typical savings are 5–40%, more for files exported by older software.
Strong – re-render pages as images (big gains). Each page is drawn at the resolution you choose and saved as a JPEG inside a new PDF. This is how you shrink a 40 MB scan to 3 MB. The trade-off: text can no longer be selected or searched, and links and form fields are removed, because every page becomes a picture. The tool warns you, and you should keep the original.
Choosing strong settings
- Resolution: 72 DPI – screen, smallest, 110 DPI – balanced (default), 150 DPI – good print, 200 DPI – sharp. Higher DPI means sharper small text and bigger files.
- JPEG quality: Low, Medium (default) or High. Medium is hard to tell apart from High on screen.
- Convert to greyscale: removes colour, which saves space for black-and-white documents like letters, forms and receipts.
A good starting point for an emailed scan is 110 DPI, Medium quality. If the result is still too big for a portal limit, step down one setting at a time and check that small print is still readable.
Which method for which PDF?
| PDF type | Best method |
|---|---|
| Report or CV exported from Word or Google Docs | Lossless |
| Scanned letters, receipts, forms | Strong, 110 DPI, greyscale |
| Photo-heavy brochure for screen viewing | Strong, 110–150 DPI |
| Contract you must sign or fill in | Lossless (keeps form fields) |
| Print-ready artwork | Do not compress; send as-is |
Why some PDFs barely shrink
If a PDF is mostly text from a modern word processor, it is already compact, and lossless optimisation can only shave a little. Large size usually comes from images: scans, photos or screenshots. Strong mode targets exactly that. Fonts can also add a few hundred kilobytes each; they are kept in lossless mode so text displays correctly.
Hitting a file-size limit
Upload portals for visas, universities, courts and job sites often set limits like 1 MB, 2 MB or 5 MB. Steps that work:
- Try Lossless. Check the new size shown under the result.
- If it is still too large, use Strong at 110 DPI, Medium. Tick greyscale for black-and-white paperwork.
- Still too big? Remove pages you do not need with Split PDF or Organize PDF, or go to 72 DPI.
- If the portal accepts several files, split the document and upload the parts.
Our guide How to compress a PDF for email explains attachment limits for Gmail and Outlook and when to send a link instead.
Compressing images before making a PDF
If you are building a PDF from photos, it is more efficient to shrink the photos first with Compress Image or Resize Image, then combine them with JPG to PDF. That way you control the quality of each image directly.
Password-protected PDFs
Encrypted PDFs cannot be optimised until the encryption is removed. Use Unlock PDF with the password, compress, then add the password again with Protect PDF.
Privacy and speed
Compression happens entirely on your device. The PDF engine (about 1.3 MB) downloads the first time you use lossless mode and is cached, so the tool also works offline afterwards. Strong mode renders page by page and shows progress; a 100-page scan may take a minute on a phone.
Frequently asked questions
How do I compress a PDF without losing quality?
Use Lossless. It rewrites the file more efficiently – compressing streams and removing unused objects – without touching text or images. Gains are usually 5–40%.
How do I make a PDF much smaller?
Use Strong. Each page is re-rendered as a JPEG image at the resolution you pick, which can shrink scanned PDFs by 70–95%. Text will no longer be selectable.
Which resolution should I choose for Strong?
110 DPI (balanced) for documents read on screen, 150 DPI for printing, 72 DPI for the smallest file, and 200 DPI when small text must stay sharp.
Why did my PDF barely get smaller?
It was already efficient – many PDFs made by Word or modern software are. Try Strong if you need a big reduction and do not need selectable text.
How do I compress a PDF to under 1 MB or 2 MB?
Try Lossless first. If it is still too big, use Strong at 110 DPI and Medium quality; drop to 72 DPI or Low quality if needed. Removing pages with Split PDF also helps.
Is my PDF uploaded to a server?
No. Compression runs in your browser using qpdf and PDF.js compiled to WebAssembly.
Can I compress a password-protected PDF?
Unlock it first with Unlock PDF using the password, compress it, then protect it again if needed.