Compress PDF without losing quality

The half that costs nothing visible

Two of the four methods leave images alone. The structural method rebuilds the document into a fresh file, leaving behind overhead that edits piled up over time. The font method subsets each embedded font down to the glyphs the pages actually draw, so a font that shipped with thousands of glyphs stops carrying the ones your document never uses.

Both keep every pixel the images had and every non-space character the text had, when the source is readable - that is what makes them the first try, not a guarantee they reach your number. Their ceiling is low: free savings are still savings, but a tight number takes the lossy half as well - and the verdict names the method that actually got there.

The half that costs something

The image method re-encodes placed images at 150 dpi and quality 75. The raster method renders whole pages at 150 dpi and picks its own quality from a sample of at most eight pages, inside a fixed range of 60 to 92. Both are lossy, and both are where a demand for a small file gets paid for.

The range itself tells the story: the top of it exists so ordinary files keep their look, and the floor exists so the method stops before mush. Whether a given file lands high or low depends on how far above the target it started - no measured distribution exists here, so judge your own result instead of the average.

150 dpi is the other lever, and it is a deliberate one. If you are compressing something that will be printed at full size and inspected closely, send the original instead: 150 dpi sits below what a print shop wants, and that is the number to think about, not the quality figure.

Text quality is not part of the trade

The one thing never traded away is the text. Even the raster method, which turns the visible page into an image, writes the page's own text back first as an invisible layer, so selection, search, copy and paste and screen readers all keep working on the compressed file.

That is enforced, not promised. Every squeezed candidate is read back and compared to the source, and one that lost more than 2% of the non-space characters is refused rather than offered. A file already under the target returns unchanged, and a source the reader cannot open has no baseline to compare.

Resolution is the setting people mean

When someone says a compressed PDF looks bad, the cause is usually resolution rather than JPEG quality. An image placed at 600 dpi and re-encoded at 150 dpi keeps one pixel in sixteen, and no quality number brings the rest back.

For that reason, a page going to a commercial printer at full size deserves the original: for that job compression is the wrong tool. Send the original.

How to keep quality in practice

Three habits protect quality more than any setting inside the tool.

1. Ask for the largest file the receiving system will take. Every KB you leave on the table is quality you keep. A form that caps at 2 MB should get a 2 MB target, not a 500 KB one chosen out of caution.
2. Cut pages before you cut quality. Removing four pages of appendix photos saves more than any quality setting and costs nothing on the pages that remain.
3. If you have no hard cap at all, the quality page runs the same engine with an empty target box, which falls back to 500 KB - or just under your file size when the file itself is smaller.

When you do have a cap, the compress tool takes it directly. The general guide covers which of the four methods your file will lean on, and the compress hub lists the rest of the cluster.