Compress a PDF to fit an upload limit

When this works well, and when it does not

PDF compression is really image compression in disguise. Most oversized PDFs are large because they contain scanned pages stored as high-resolution photographs. Reducing the resolution and re-encoding those pictures is where nearly all the saving comes from.

• Works very well: scanned documents, photographed forms, PDFs exported from a phone scanner app, presentations with large photos.
• Works poorly: PDFs that are mostly text created directly from Word or LaTeX. Those are already small, and rebuilding them as images can make them larger while losing searchable text.

How the target size mode works

Rather than making you guess at DPI and quality settings, target mode renders your pages once and then searches for the highest quality that still lands under your limit. If quality alone cannot get there, it steps the resolution down and tries again.

This means it takes longer than picking settings by hand, particularly on long documents, but it removes the usual cycle of compressing, checking the size, and compressing again.

Choosing a resolution manually

Greyscale is worth trying on any black-and-white text scan. Colour information on a page that has no colour costs bytes for nothing, and switching it off often cuts the file by a third or more.

Related things people need

If your PDF started life as a set of photographs, you will get a better result building it properly in the first place. The images to PDF tool lets you set the resolution as it assembles the document, rather than compressing away detail afterwards.

To go the other direction and pull pages out as pictures, use PDF to JPG.

Why PDFs get large

A PDF is a container, and its size depends almost entirely on what has been put inside it. A text document exported from Word is typically a few hundred kilobytes regardless of length, because text is stored as instructions to draw characters — extraordinarily compact.

A scanned document is the opposite. Each page is a photograph, stored at whatever resolution the scanner produced, often 300 or 600 dots per inch. A single scanned page at 600 DPI in colour can exceed 20MB on its own. A twenty-page scan becomes a file no email will carry.

Everything this tool does follows from that distinction. It works on the pictures, because in an oversized PDF the pictures are essentially the whole file.

What DPI actually means here

Dots per inch describes how finely a page is sampled. At 300 DPI, a letter-sized page is captured as roughly 2550 by 3300 pixels — around eight megapixels for a single sheet of paper. At 150 DPI it is a quarter of that, and at 100 DPI a ninth.

The right choice depends entirely on what happens to the document next. Reading on a screen needs remarkably little: 100 DPI is comfortable, because a screen displays around 100 pixels per inch anyway. Printing is where higher resolution earns its size, since print reproduces far finer detail than a display.

Most people compressing a PDF are sending it to be read, not printed, which is why 150 DPI is the default here. It looks clean on any screen and produces files a fraction of the original.

Greyscale is the underrated setting

A black-and-white document scanned in colour stores three colour channels for every pixel, describing colour information that is not there. Converting to greyscale discards two of those channels and commonly removes 30 to 50 percent of the file with no visible difference whatsoever.

It is worth trying on any text document, contract, form or letter. The only reason not to is if the page genuinely contains colour that matters — a highlighted passage, a coloured logo, a signature in blue ink you need to prove was blue.

The trade-off you are making

This tool rebuilds each page as an image, and that has a real consequence worth being clear about: any selectable text stops being selectable. You cannot search the document, copy a quotation from it, or have a screen reader read it aloud.

For a scanned document that never had selectable text, nothing is lost. For a PDF exported from Word, you are trading away something genuinely valuable — and usually getting little in return, since text-based PDFs are already small.

The rule of thumb: if you can select text in the original by dragging across it, think twice. If you cannot, it was already a picture and compression is pure gain.

Questions

Will my text still be searchable afterwards?

No. Because each page is rebuilt as an image, text selection, copying and search stop working. If you need searchable text, keep the original and send that instead.

Is my document uploaded?

No. The whole process runs in your browser. This matters more for PDFs than for most files, since they often contain identity documents, medical records and financial statements.

Why did my PDF get bigger?

Your original was probably a text-based PDF rather than a scan. Those are already efficiently stored, and converting them to images adds bytes. Keep the original.