Why is my PDF so big? How to reduce PDF file size
Find out what makes a PDF large, pick the compression method that fits the document, and recognise when a file cannot get much smaller.
A PDF can be a few kilobytes or hundreds of megabytes for the same number of pages. The difference is almost never the text. It is the images inside, and how they were saved. Once you know what is making a file large, you can choose a method that shrinks it without ruining it.
What makes a PDF large
- Photos at camera resolution. A 12-megapixel photo placed on an A4 page holds far more pixels than any screen or office printer needs.
- Scans. Each scanned page is one big image. Scanning in colour at 300 or 600 DPI creates large files quickly.
- Embedded fonts. Usually small, but documents with many complete font files can add up.
- Inefficient structure. Some programs save PDFs in a verbose way. This adds a little, rarely a lot.
Three methods, and when each one helps
| Method | What changes | Best for |
|---|---|---|
| Recompress photos | JPEG photos are saved again at lower quality and, at stronger levels, at a smaller size. Text and drawings are untouched. | Brochures, reports and presentations with photos. |
| Lossless clean-up | The internal structure is packed more tightly. Nothing visible changes. | When no change at all is acceptable. Expect small savings. |
| Convert pages to images | Each page becomes one JPEG. Text is no longer selectable or searchable, and links stop working. | Scans and photo-only documents that must get much smaller. |
Choosing a level
Start with the middle level and open the result at 100% zoom. Look at photos with fine detail and at small text in images. If it looks the same, try the stronger level; if you see blocky areas, go back. For documents that will be printed, stay at the light level or keep the original.
When nothing helps
If a PDF is mostly text, its size comes from fonts and structure, and there is little to remove without changing the document. Images saved losslessly (PNG-style) or in CMYK, which is common in print-ready files, are also left alone by the photo method, because re-encoding them in the browser could change colours. In those cases the honest answer is that the file is already about as small as it can be. Splitting it into parts, or sending it as a link, are the practical alternatives.