File optimisation
How to compress a PDF without ruining it
6 min read
Most oversized PDFs are not oversized because of their text. They are large because of the images inside them. Understanding that changes how you approach compression — and stops you destroying a document to save a megabyte.
Find out what is making the file big
A hundred pages of pure text is typically well under a megabyte. If a five-page PDF is 20 MB, it almost certainly contains full-resolution photographs or a page-per-page scan. Scroll through the document: if you can zoom deeply into a page and still see clean detail, the source images are far larger than they need to be.
Compress the images before you build the PDF
The most effective order is to shrink the inputs, not the output. Resize photographs to the size they will actually be viewed at, compress them to 75–85% quality, and only then assemble the PDF. A document built from optimised images is smaller and sharper than the same document crushed afterwards.
Remove what you do not need
- Delete blank scan pages, cover sheets and duplicated appendices.
- Extract only the pages the recipient actually asked for.
- Split a very long report into logical parts instead of sending one huge file.
Sensible targets
- Email attachment: under 10 MB, ideally under 5 MB.
- Web download: under 2 MB for anything you expect people to read on mobile data.
- Print submission: follow the printer's specification rather than compressing blindly.
What compression cannot fix
If a scan is blurry, compressing it will not help and re-scanning will. If a PDF contains an embedded video or a hundred high-resolution product photos, no compression setting turns it into a small file — the content has to change.
Frequently asked questions
Does compressing a PDF lose quality?
Compressing the images inside it does, because image compression is lossy. Removing pages or rebuilding a document from smaller images does not degrade the pages you keep.
Why is my scanned PDF so large?
Scanners often save each page as a high-resolution image. Reducing the scan DPI to 200–300 for text documents dramatically cuts size.
Is there a size limit on these tools?
The interface accepts files up to 100 MB, with the real limit set by your device memory.
Treat PDF size as an image problem first, a page-count problem second, and a compression-slider problem last. That order gives you the smallest file with the least visible damage.