Compress a PDF without making it look terrible
How DPI, JPEG quality, and re-encoding decide whether your compressed PDF still looks professional.
Every "compress" tool offers a slider and zero explanation of what it's doing to your file. Here's the short version of what to leave alone and what to drop.
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The two knobs that matter inside a PDF compressor are image DPI and JPEG quality. Tools dress them up with names like "Smart", "Balanced", "Small", but underneath it's the same two dials.
What each knob does
DPI controls how many pixels per inch each embedded image keeps. A 600 DPI scan downsampled to 150 DPI is one-sixteenth the data. On screen at 100% zoom, the difference is invisible; printed at A4 it's still sharp enough. Below 100 DPI you start seeing softness.
JPEG quality decides how aggressively each image is re-encoded. 90% is visually lossless. 75% is fine for screen viewing. Below 60% you'll start to see compression artifacts on solid-colour backgrounds.
When to bail and use a different tool
If your PDF is mostly vector art — diagrams, schematics, exported InDesign — compressing it does almost nothing because there are no images to downsample. The honest move is to re-export the source with a smaller embedded-font set or split the file.
If the PDF was already aggressively compressed once, re-compressing introduces visible artifacts without saving much space. You can't squeeze a sponge twice.