Why is my PDF so large, and how do I make it smaller?

Updated 16 October 2026

You try to upload a PDF to a job portal or attach it to an email, and it’s rejected for being too large. The frustrating part is that two PDFs with the same number of pages can be wildly different in size. This guide explains why, and what to do about each case.

What actually takes up space in a PDF

A PDF can hold three kinds of content, and they take up very different amounts of space.

1. Scanned pages and photos (usually the cause)

When you scan a document or photograph it with your phone, each page is saved as a picture. A picture stores every dot of colour on the page, including the blank paper, so a single scanned page can be larger than a whole typed report.

Two things make scans even bigger:

• High resolution. Many scanners default to 300 DPI or more. That is useful for printing photos, but far more than you need to read a letter on screen.
• Colour. A colour scan of a black-and-white letter stores a lot of colour information that adds nothing.

You can tell a page is a scan if you can’t select individual words in it.

2. Fonts

PDFs usually carry the fonts they use, so the document looks the same on every device. Most fonts are small, but some are large, especially fonts with many characters or several styles. Fonts can’t be shrunk without changing how the text looks, so a long document with heavy fonts has a size floor.

3. Text and drawings

Typed text, lines and tables are stored as instructions (“draw these letters here”), which are tiny. A document saved directly from Word, Google Docs or a website is mostly this kind of content, and that is why it is usually small already.

How much can a PDF shrink?

It depends almost entirely on how much of the file is pictures. These are real results from documents we tested with PDFHive’s Compress PDF:

The pattern is clear: scans shrink a lot, while documents made of typed text barely change because there is little left to remove.

How to make your PDF smaller

Work through these in order. Most files are fixed by the first step.

1. Compress it. Open Compress PDF and choose Recommended. It scales scanned pages down to about 150 DPI for an A4 page, which still looks sharp on screen and prints well. You see the new size before you download anything.
2. Go further if you need to. Choose Strong (about 105 DPI) for the smallest file that still reads clearly. If a website gives you a limit, choose Target size and type it in, for example 500 KB. There are ready-made pages for common limits such as Compress PDF to 100KB.
3. Use Black and white for text-only scans. For scanned letters, statements and forms, Black and white often makes the file several times smaller again, with crisp text. Don’t use it for photos, ID cards or pages with coloured stamps you need to keep, because it removes all colour.
4. Send fewer pages. If a form only needs the first page or two, keep just those with Split PDF, or remove the rest with Delete PDF pages. Fewer pages is the one fix that works for every kind of PDF, including ones that are mostly fonts.

Avoiding large PDFs in the first place

If you scan documents often, a few settings save a lot of trouble later:

• Scan text documents at 150 to 200 DPI rather than 300 or more.
• Choose greyscale or black and white for documents that have no colour.
• Scan only the pages you need, and save each document as its own PDF.

When a PDF won’t get smaller

If Compress PDF says your file is already well compressed, it is mostly text and fonts, and there is nothing it can remove without changing how the document looks. In that case, cutting pages is the only real option. Converting a typed PDF into images (for example by “printing” it to a new PDF as pictures) usually makes it larger, not smaller, so it’s best avoided.