Compress PDF

Almost all of the weight in a large PDF is pictures. A twenty megabyte file is nearly always a scan, and a scan is a stack of photographs of paper. Compressing it means storing smaller photographs.

Everything happens inside this page. Your file is read by JavaScript running on your own machine, and no part of it is sent anywhere.

What this tool actually does, and what it costs

Each page is drawn at the resolution you choose, saved as a JPEG at the quality you choose, and placed on a page of the original size. The document still opens, still prints and still has the same page count and dimensions.

The cost is that text stops being text. If your PDF came from Word and has selectable, searchable words in it, they become part of a picture and are no longer selectable or searchable. That is a genuine loss and it is why this tool is marked with a caveat rather than presented as free of charge.

So the rule is simple. On a scan there is nothing to lose, because the text was already a picture, and the saving is enormous. On a document that was generated from a word processor, do not use this: it is usually already small, and compressing it makes it worse in a way that matters.

Choosing a resolution

150 dpi is the recommended setting and it is not a compromise. It is the resolution most office scanners produce and it prints cleanly on any normal printer. A scan that arrived at 300 dpi will roughly quarter in size at 150 with no visible difference on paper.

100 dpi is for documents that will only ever be read on a screen. It is noticeably softer in print but perfectly readable on a display, and it is the setting for getting something under an email limit.

200 dpi keeps more detail and is worth it when the pages have small print, tables of figures, or handwriting on them.

Quality is a separate dial

Resolution decides how many dots are stored. Quality decides how hard each of them is squeezed. Both affect size and they do different damage.

Around 70 to 80 per cent is the range where JPEG artefacts stay invisible on a document. Below 50 the edges of letters start to smear, which shows up worst on exactly the small text you were trying to keep readable.

If the file barely shrinks

That means it was not picture-heavy to begin with, and there was nothing much to compress. A text document from a word processor is mostly font data and instructions, both of which are already compact.

In that case leave it alone. A file that is already small is not a problem, and the honest answer is better than pretending an operation helped when it did not.