“Attachment too large” usually comes up at the worst moment. Gmail takes attachments up to 25 MB, Outlook usually 20 MB, and many company mail servers stop at 10 MB. A single scanned document can be bigger than all of those.
Why some PDFs are so big
Text in a PDF is tiny: a hundred pages of typed text can fit in less than a megabyte. What makes PDFs heavy is pictures: scanned pages, photos, screenshots. A page scanned in colour at 300 dpi is a picture of roughly 2500 × 3500 pixels, and a phone photo pasted into a document can be even larger.
That’s also why compressing a PDF doesn’t have to touch its text. The size is in the pictures, so that’s where it should be saved.
How to compress it
- Open the PDF in Compress a PDF.
- Pick a level:
- Light: pictures down to 200 dpi. Best quality, smaller savings.
- Recommended: 150 dpi. Sharp on screen and fine for printing everyday documents.
- Strong: 110 dpi at lower quality. The smallest file, still readable.
- Check the result: you see the size before and after.
- Download the smaller PDF.
Only pictures sharper than the chosen level are made smaller. Text, lines and vector drawings are left exactly as they were, so they stay crisp at any zoom. A typical scanned document shrinks to about a third of its size on Strong.
Still too big?
- Split it. Send the pages that matter: open the file in Organize pages, select pages and extract them into a new PDF.
- Scan in greyscale. If you scan it yourself, greyscale at 150–200 dpi is plenty for documents and much smaller than colour at 300 dpi.
- Don’t compress twice. Compressing an already compressed file saves little and costs quality. Start from the original.
Is it safe to compress confidential files?
Here the file is compressed in your browser, on your device. It is never uploaded, so it works for contracts, payslips and medical documents just as well as for anything else.