Why your PDF is so big, and how to make it smaller

Email servers often refuse attachments above 20 or 25 MB, and many upload forms cap files at 5 or 10 MB. When a PDF is too heavy, the instinct is to look for a "compress" button. Before you do, it helps to know where the weight actually comes from, because the right fix depends on it.

What takes up space in a PDF

• Text is almost free. A page of plain text is a few kilobytes, even with the font embedded.
• Vector graphics, charts and logos made of lines and shapes, are small too.
• Photos are where size lives. One uncompressed photo can outweigh a hundred pages of text.
• Scans are photos of every page, so a scanned document is all photo.
• Fonts occasionally matter: a document using six full font families can carry a few megabytes of font data.

Fix it at the source if you can

The best compression happens before the PDF exists. If you made the file yourself:

• In Word or Pages, resize photos to the size they appear on the page before exporting. A 4000-pixel image shown 8 cm wide is wasted weight.
• When exporting, choose a "smaller file" or "standard" option rather than "best for printing" unless it really is going to a printer.
• For scans, scan at 150 or 200 dpi in greyscale rather than 600 dpi in colour. For text documents it's indistinguishable on screen.

When you only have the PDF

Real compression means re-encoding the images inside the file and leaving the rest alone. That's what Compress PDF does: it finds every photo and scanned page, brings it down to a resolution that still prints cleanly, re-saves it as a JPEG and throws away objects nothing points to. Text, fonts and vector drawings are left untouched, so the result stays sharp and searchable.

Start with Recommended. If the file is a scan and still too heavy, Maximum redraws each page as a single image, which shrinks it the most but makes the text unselectable. If compression barely changes anything, the weight isn't in the images, and these work better:

• Drop what you don't need. Sending three pages of a 60-page report? Extract those pages and the file shrinks with them.
• Remove blank pages from scans with Remove blank pages.
• Convert a colour scan to black and white with Black and white PDF at 150 dpi. For a text-only scan this often cuts the size several times over.
• Use your computer's print dialog. On a Mac, open the PDF in Preview and use File → Export with the Reduce File Size filter. On Windows, "Microsoft Print to PDF" re-creates the file, sometimes much lighter.
• Share a link instead of an attachment if the recipient has a shared drive.

A quick sanity check

Divide the file size by the number of pages. Under 100 KB per page is normal for text documents. 300 KB to 1 MB per page is typical for scans. Several megabytes per page means images at a resolution nobody will ever see, and that's the file worth fixing at the source.