How to Reduce PDF File Size Without Losing Quality
A ten-page contract with no images might be 200KB. A ten-page scan of that same contract, photographed instead of exported, can hit 40MB. Almost all of that weight is images, not text — which is the key to compressing smartly instead of just cranking a slider and hoping.
What's actually happening during compression
Two things, mostly: images get downsampled to a lower resolution, and whatever's left gets compressed harder. Text and vector graphics are usually left alone entirely, since they barely take up space to begin with.
Picking a compression level
Light compression is plenty for anything that'll only be read on a screen — most monitors can't even show more detail than a 150 DPI image gives you. Medium is a safe default for general sharing. High is for situations where size matters more than anything else and you don't need to print it, because it will visibly soften photos and scanned text.
Scanned documents specifically
Scanners default to 300 or 600 DPI more often than not, which is way more than you need for reading on a screen. A compressor built for scans specifically can often cut 80% off the file size with barely any visible difference.
What not to do
Don't run the same file through compression twice. Each pass re-compresses already-lossy images, and the damage stacks. If a compressed file is still too big, go back to the original and try a stronger setting from scratch — don't compress the compressed version.
A number to aim for
Most email providers cap you around 25MB, and plenty of upload forms are stricter than that. Know your target size before you start rather than compressing three times by trial and error.
