How to compress a PDF without losing quality

“Without losing quality” is not a slider labeled Magic. A PDF stores two very different things: vector text (operators that say “paint these glyphs here”) and sampled images (JPEG/JPEG2000/Flate bitmaps). Quality loss almost always means you re-encoded the bitmaps. Font programs and text operators do not get “more compressed” in a useful way; they are already small.

What actually gets removed

Embedded image resolution. A desktop scanner at “Color, 300 dpi, full page” writes roughly 2550×3300 pixels per Letter page. JPEG quality 90 on that raster is often 1.2–2.0 MB per page. Twenty-eight pages is 34–56 MB before anyone emails it. Re-encoding those streams at a lower JPEG quality — which is what Compress PDF does on Low and Recommended — is the entire game. Text stays selectable because we do not delete the content stream that paints it.

Font subsetting is usually already done by Word, InDesign, or the print driver. A second subset pass rarely saves more than tens of kilobytes. Metadata (Author, Title, XMP) is kilobytes, not megabytes. Stripping it does not get you under Gmail’s cap. If someone sold you “compress by removing metadata,” they were talking about a 200 KB file.

Worked example

28-page signed lease, phone-scanned, 42.6 MB. Almost every page is one full-bleed JPEG. Recommended typically lands in the 6–9 MB band — still readable at 100% zoom, text still highlightable if the OCR layer or typed text was there; on a pure scan there was never selectable text to lose. High can land under 3 MB by rasterizing each page again as a single image. You can no longer copy a clause. That is the quality you lost: not “a bit of grain,” the text layer itself.

Counter-example: a 12-page policy exported from Word, 184 KB, subset fonts, no photos. Running Recommended may come back at 190–210 KB. You added a rewrite. High would balloon it: twelve page-size JPEGs instead of a few kilobytes of vectors. That is High backfiring on a text-heavy document.

When High is the wrong button

Use High when a portal still refuses Recommended and you do not need search. Do not use High on a contract you still intend to quote from. Do not use High on a fillable form if you need the fields. Flattened pages are pictures of forms, not forms.

If the file is large because it has 80 pages and you need three of them, split first. Compressing pages you are about to delete is wasted RAM.