How to compress a PDF
Understand what actually makes a PDF large, and pick a setting that fits your purpose.
Almost every oversized PDF is oversized for one reason: the images inside it are stored at a far higher resolution than anyone will ever look at. A phone photograph of a receipt might be twelve megapixels. Printed on a page, it is perhaps one.
Compression works by resampling those images down to a resolution that still looks right at the size they are displayed. That is why the presets are described in DPI rather than as vague quality levels — DPI is the thing being changed.
The steps
- Upload the document.
- Choose a preset: Light (200 DPI), Balanced (150 DPI), Strong (100 DPI) or Extreme (72 DPI).
- Run the tool and compare the real before and after sizes.
- Download it, or try a stronger preset if it is still too big.
What the four presets mean
Light works at 200 DPI and is the setting to use if the document will be printed, where image detail still matters. Balanced at 150 DPI is comfortable for reading on screen and is the sensible default. Strong at 100 DPI produces a clearly smaller file with visible softening in photographs. Extreme at 72 DPI matches typical screen resolution and is for when the size limit is the only thing that matters.
Text and vector graphics are never rasterised at any preset. Your text stays sharp and selectable even at 72 DPI, because only the embedded images are resampled.
Why some files barely shrink
A document that is mostly text was never large because of images, so resampling images cannot make it much smaller. If you compress a text report and save two percent, that is not a failure — there was simply nothing to reclaim.
For those files, try Optimize instead. It is lossless and targets a different kind of waste: orphaned objects, uncompressed streams and redundant data left behind by whatever produced the file.
Check the real numbers
After a run you are shown the actual before and after sizes of your specific file. Treat any tool that promises a percentage in advance with suspicion — the honest answer always depends on what is inside your document.
