How to make a PDF smaller, and when you shouldn't

A text PDF (an invoice, a report, a CV exported from Word or Google Docs) stores letters, fonts and positions. Ten pages of it can be a few kilobytes. A scanned PDF, or one made from phone photos, stores each page as a photograph of the paper. A single page can be a megabyte or more, and that is where size problems come from.

Compress PDF draws every page as an image and saves that image as a JPEG at a lower resolution. The three levels mean:

• High quality: pages at 144 pixels per inch, JPEG quality 82.
• Balanced: 108 pixels per inch, quality 68.
• Maximum compression: 79 pixels per inch, quality 50.

All three read comfortably on a screen. For printing, 144 is acceptable and 79 is visibly soft. Everything happens in your browser; the PDF is not uploaded.

We ran three PDFs through each level: three pages of text stored as 300-dpi images (the way a scanner or a phone scanning app saves them), three pages each holding a 12-megapixel photo, and a ten-page text document exported the way a word processor does it.

The scanned pages fell to a tenth of its size on Maximum, and the photo pages to a twentieth. The text document went the other way: 10 KB became 564 KB at best and 2.2 MB at worst, because ten pages of compact text were replaced by ten pictures of text. In that case the tool now tells you to keep your original. You can still save the larger version if you really want image-only pages, but it is never the default.

Because every page becomes a picture, the compressed PDF loses:

• selectable and searchable text, and with it copy-and-paste;
• links, form fields and anything else interactive;
• screen-reader access, since there is no text left to read aloud;
• sharpness when printed, especially on Maximum.

For a scan none of this matters, because there was no text layer to begin with. For anything else, keep the original as well.

• Email: start with Balanced. Most mail services cap attachments at 20 to 25 MB, and Balanced brings typical scans well under that.
• A portal with a hard limit: try Maximum, then open the result and read the smallest print on it. If it is still legible, upload it.
• Printing or archiving: don't compress. Keep the original.

• Send fewer pages. Delete pages the recipient doesn't need, or extract just the ones they asked for.
• Send it in parts. Split the PDF when a portal allows several uploads but limits each one.
• Fix scans at the source. A black-and-white scan of a text page is far smaller than a colour photo of it, and easier to read.
• For a text PDF that is still too big, the cause is usually embedded images or fonts. Re-export it from the program that made it with that program's smaller-file option, rather than compressing the PDF afterwards.