How to compress a PDF, and what actually makes them big

What actually makes a PDF big

Almost always, it is images. A page of text is tiny, a few kilobytes of drawing instructions. A single photograph at print resolution can be several megabytes on its own. When a PDF is unexpectedly large, the cause is nearly always one of these:

• Scanned pages. Every page is a full-page photograph. A 40-page scan is 40 large images, and this is the most common reason by a wide margin.
• Images placed at far higher resolution than needed. A photo straight from a phone camera dropped into a document keeps all 12 megapixels even though it is displayed two inches wide.
• Embedded fonts. Usually modest, but a document embedding several full font families rather than subsets adds up.
• Retained revision history. Some editors append changes rather than rewriting the file, so a heavily edited document can carry versions of content you deleted long ago.

Knowing which one applies to your file tells you whether compression will help. If your 8 MB document is 8 MB because it is a scan, compression will help enormously. If it is 8 MB because it contains one enormous embedded font and forty pages of text, it will barely move.

How compression works here

Be clear about the method, because it determines the trade-off. Each page is rendered to an image, that image is encoded as a JPEG at a chosen resolution and quality, and a new PDF is built from those images.

That is a blunt instrument, and it is very effective on the case that actually causes large files. It also means the output is a document of pictures. This is the single most important thing to understand about compressing a PDF, and it is covered properly below.

Compressing a PDF

1. Open the PDFLocally, as always. Nothing is uploaded.
2. Choose Compress from the toolbarYou are asked to pick a quality preset.
3. Pick a presetSee the guidance below. If unsure, Balanced is the sensible default.
4. Check the resultA new file downloads with -compressed appended to the name, and you are told the before and after sizes and the percentage saved.

Your original is untouched on disk and the working document in the editor is unchanged, so you can try a different preset immediately if the first one was too aggressive.

Choosing a preset

• High quality renders at double resolution with light JPEG compression. Sharpest output and moderate savings. Use it when the document will be printed or read closely, or when it contains small text or fine detail.
• Balanced renders at one and a half times resolution with moderate compression. The right default for a document that will be read on a screen and emailed around.
• Smallest file renders at just above screen resolution with heavy compression. Use it when a hard size limit is the only thing that matters, for instance an upload form that rejects anything over 2 MB. Expect visible artefacts on close inspection.

Start one step gentler than you think you need and check the output. Compression artefacts are much easier to see on the actual document than to predict from a description.

What you give up

The other cost is that image quality loss is permanent. JPEG is lossy, and compressing an already compressed page again compounds the damage. Always compress from the original rather than re-compressing a compressed file.

If your goal is images rather than a smaller PDF, exporting pages directly as images is a better route than compressing and then extracting. That is covered in the image export guide.

When compression will not help

If the output would be no smaller than the input, you are told so and no file is produced. The message says the PDF is already compact, and it is worth believing rather than trying a harsher preset out of stubbornness.

This happens for genuine reasons. A text-only document is already close to as small as it can be, and rasterising it makes it larger, not smaller. A file that has already been compressed once has little left to give. In both cases the right answer is that the file is not the problem, and you should look at whether it really needs to be smaller.

One case worth flagging: a document containing redactions is already partly rasterised, because redacted pages are flattened to images so the covered text is genuinely removed. Those pages are larger than text pages by design. Compressing afterwards will shrink them, and it will not undo the redaction, which remains permanent. See the redaction guide for what that flattening actually does.