What compression actually does to your files

“Compress this file” hides three completely different operations behind one verb. Knowing which one a tool performs is the difference between a file you can rely on and one that fails in front of an examiner or a client.

The three kinds of compression

1. Lossless re-packing. The file’s data is stored more efficiently (better containers, object streams in PDFs) but every pixel and glyph is preserved exactly. This is what our merge, split and unlock tools do. Result: files get modestly smaller, at worst, and quality is untouched by definition.

2. Lossy re-encoding. Data the eye weighs least is discarded, then the remainder is stored more efficiently. JPEG/WebP images and rasterised PDF pages go through this. A 20× reduction in size is routine. The trade-off is real and visible at high zoom — there is no free lunch, and tools claiming “no quality loss ever” are either lying or doing type 1.

3. Resolution reduction. Fewer pixels, period. Crops fine for screens and forms, loses detail for print. This is usually combined with type 2 as the second lever, and a good tool tells you when it pulled that lever — ours does, right on the result card.

How to verify a compressed file in 60 seconds

Check the bytes are what was claimed. Right-click → properties. Any tool that quotes sizes should match the file you actually got. (If they differ, the tool estimated instead of measuring — a red flag.)
Open it at 100% zoom, not fit-to-window. Fit-to-window hides everything; 100% shows blocking around sharp edges, smearing in textures, halos around text.
For forms: check what the verifier reads. Portal moderation usually zooms into faces and signatures. What passes at thumbnail size can fail the zoom test.
For scans: confirm text is still text. If a “compressed PDF” used to let you select text and now doesn’t, the tool rasterised it silently. (Our PDF compressor says exactly that on the card when it happens, because silent rasterisation is a quality event you should be choosing, not discovering later.)

Which kind do you need?

Match the operation to the situation:

Sending for viewing (email, chat, listings): lossy is fine and expected.
Uploading to forms: lossy within their stated band, verified at 100%.
Archiving, court documents, anything re-edited later: lossless only — split, merge, and password changes, but never rasterise.

If a tool will not tell you which of the three it performs, assume the one that makes its numbers look best.