A 40-page PDF of mostly text should be a few hundred KB. If yours is 40MB instead, it's almost always one specific culprit: images. Scanned pages, embedded photos, and high-resolution graphics are what actually drive PDF file size - the text itself barely registers.
What's actually taking up the space
- Scanned pages. A scan saved at a high DPI (dots per inch) captures far more detail than a screen or even most prints need, and that detail adds up fast across many pages.
- Embedded images. Photos or graphics pasted in at their original resolution, when the PDF only displays them at a fraction of that size.
- Embedded fonts. Less common as a major factor, but custom fonts embedded for every character style used can add real weight to a design-heavy document.
What compression can realistically do
Being straightforward about this rather than overselling it: real, dramatic PDF compression (cutting a file to a tenth of its size with no visible quality loss) generally requires actually re-encoding the images inside the PDF at a lower resolution or quality - something that needs a full PDF-rendering engine like Ghostscript. A browser-based tool that re-processes the PDF's internal structure without that kind of image re-encoding will give you real, modest savings - not the dramatic shrinkage some tools promise. If you see a tool claiming to cut any PDF by 90% with zero quality loss, be a little skeptical of the "zero quality loss" part specifically.
Try Compress PDF and compare the before/after size - for many PDFs (especially ones with redundant structure or metadata) it's a genuine, worthwhile reduction, just not a guaranteed miracle for every file.
Getting a smaller file from the start
The most effective fix often happens before you ever get to the compression step:
- Scan at a lower DPI if the document is just for reading, not printing. 150-200 DPI is plenty for on-screen viewing; 300+ DPI is really only necessary if it'll be printed at high quality.
- Resize images before you build the PDF. If you're placing a 4000px-wide photo into a document where it displays at 400px, you're carrying 10x the file size for zero visible benefit. Resize Image first, then build your PDF.
- Avoid re-scanning already-scanned documents. Each generation of scan-of-a-scan tends to add artifacts and sometimes gets saved at an unnecessarily high resolution again.
Doing it
- Open Compress PDF
- Upload your file
- Download the result and compare the size to the original
If the size difference isn't dramatic, that's an honest reflection of what's achievable without a full rendering engine - see the note above rather than assuming something went wrong.