How to reduce a PDF's file size

What makes a PDF large

Text is tiny. A hundred pages of plain text is often well under a megabyte, because each character is stored once and drawn with an embedded font. Vector drawings, such as charts and lines from Word or Excel, are small too.

Images are what grows a PDF. A single phone photo can be 3 to 5 MB, and a scanner set to 600 DPI colour produces pages that are each a large photo. Many PDFs also carry images at far higher resolution than anyone will see: a 4000-pixel photo placed in a 5 cm box on the page is still stored at 4000 pixels.

Fonts add a little, especially when the whole font is embedded rather than only the characters used. Forms, comments and attachments add a little more.

1. Compress the images

This is the method that makes the biggest difference. A compressor re-encodes each image at a sensible resolution and quality and leaves text and drawings exactly as they are, so the document still reads perfectly.

In PDF Workdesk's Compress PDF there are three levels. Light keeps images up to 2600 pixels on the long side, for documents you'll print. Balanced keeps up to 1800 pixels and suits email and sharing. Strong keeps up to 1200 pixels, which is still clear on a screen and gives the smallest file. It runs in your browser, so the PDF isn't uploaded.

2. Remove pages you don't need

Blank pages from a duplex scan, cover sheets, appendices: each scanned page you drop is a whole image gone. Delete pages lets you click them away and keeps the other pages untouched.

3. Split it into parts

If a portal has a hard per-file limit, splitting a long PDF into two or three files may be the only way through without lowering quality. Split PDF can cut it by page ranges or into parts of a set number of pages.

4. Re-export from the original

If you have the original Word, PowerPoint or InDesign file, exporting again with a smaller-size setting often works better than compressing afterwards. In Word for Windows, Save As PDF offers "Minimum size (publishing online)". In PowerPoint, compress the pictures in the deck before exporting. In InDesign, choose the Smallest File Size preset.

5. Scan at a sensible resolution

For documents you'll read on a screen or print on an office printer, 200 to 300 DPI is plenty, and greyscale is far smaller than colour for black-and-white pages. Fixing the scanner setting once saves compressing every file afterwards.

What won't help much

• Zipping the PDF: images inside a PDF are already compressed, so a ZIP is usually only slightly smaller.
• Flattening: it locks forms and comments into the page, but rarely changes the size noticeably.
• Converting to PDF/A: it embeds full fonts and colour profiles, so the file usually gets a little bigger, not smaller.