The two kinds of content inside every PDF
Every PDF mixes two fundamentally different things. Text and vector graphics are stored as instructions — "draw the letter A in this font at this position" — which take almost no space and render razor-sharp at any zoom. Images (photos, logos saved as bitmaps, and every page of a scanned document) are grids of pixels, and they're where virtually all the megabytes live.
This is the single most useful fact about PDF size: when a file is big, it's the images. When compression "ruins" a file, it's the images. Text is essentially a bystander.
Lossless tricks: free savings
Some size reductions cost nothing in quality: removing metadata, deduplicating embedded fonts and objects, and repacking the file's internal structure (object streams). These typically save a few percent — more on bloated files from certain generators. Our Auto mode applies this kind of optimization: if your file barely shrinks on Auto, it was already efficiently packed.
Lossy compression: the quality dial
Real size reduction on image-heavy files comes from two lossy levers:
- Resolution (DPI): how many pixels represent each inch of page. Dropping a 300 DPI scan to 150 DPI quarters the pixel count. Text remains readable surprisingly far down; fine print suffers first.
- JPEG quality: how aggressively each image is approximated. High settings are visually indistinguishable from the original; very low settings show blocky artifacts, especially around sharp edges like letters.
Compressors combine both levers. Our Gentle / Balanced / Strong levels are presets along this dial — Gentle protects image quality and accepts a bigger file; Strong chases the target and accepts visible softening on scans.
What this means in practice
- Typed contract exported from Word: compresses dramatically, looks identical. Compress fearlessly.
- Resume with a headshot: text stays sharp; the photo may soften slightly at strong settings. Balanced is usually invisible.
- Color scan of a certificate: the whole page is one image — this is the file where level choice genuinely matters. Start Gentle, inspect, step down only as needed.
- Photo album PDF: every page is a photo; expect a visible (if modest) quality trade at small targets.
Choosing a compression level with confidence
Work backwards from the requirement. If a portal demands 500KB, that constraint picks the dial position for you — your job is just to verify the result is readable (zoom to 150%, check the smallest text and any signatures). If there's no hard limit, Gentle or Auto preserves the most quality while still trimming waste. And if hitting the target would make pages unreadable, the honest answer is to split the document or reduce its page count — our compressor tells you when a target can't be reached at readable quality rather than silently destroying your file.