Compress PDF

Most of a PDF’s size is its pictures: photos, scans and screenshots. Text and drawings take very little room, so they are left exactly as they are, together with links, bookmarks, form fields and the invisible text layer of a scanned page. Only the pictures and the way the file is packed change.

Each picture is looked up on its page to see how big it is drawn, then brought down to that size if it has more pixels than the level asks for, and saved again as a JPEG. A photo stored as pixels becomes a JPEG too, while flat graphics such as charts, logos and signatures stay lossless and are packed tighter. A picture that would come out larger stays as it was.

The rest of the file is packed again: its objects go into compressed groups, identical pictures and fonts become one (a logo repeated on every page is stored once), and page thumbnails are left out, and so is the editing data some design apps keep inside a PDF (Illustrator keeps a copy of the whole artwork there), so a PDF saved from Illustrator can no longer be opened again as an editable Illustrator file.

Small keeps pictures sharp enough to print: a picture drawn at more than 375 dpi comes down to 300, saved at high quality. Smaller, the default, suits screens and email: pictures over 190 dpi come down to 150, at good quality. Smallest brings pictures over 120 dpi down to 96 at lower quality, which reads well on a screen and looks soft on paper. A picture closer to the level’s resolution than that keeps its size, and only its quality changes.

One phone photo on three A4 pages (5.2 MB) comes out at 1.2 MB, 193 KB and 56.8 KB on the three levels. A form scanned at 300 dpi (3.2 MB) comes out at 1 MB, 134 KB and 56.5 KB. A report with a logo on every page, a chart, a photo and a signature (4.7 MB) comes out at 260 KB, 46 KB and 20.7 KB.

A PDF with no pictures has little to give up, and how much it gets smaller depends on how it was written. Our 30-page text report (360 separate link objects, fonts not embedded) went from 87.1 KB to 23 KB on every level, with every word, link and bookmark in place, mostly because its many small objects were packed into compressed groups. A PDF that is already packed that way changes little, and comes back marked as already as small as we can make it.

Choose Under a size, type a limit in KB or MB, and Compress looks for the gentlest settings that fit: the highest resolution and quality that still end up under it. A PDF that fits already keeps its pictures untouched.

If a PDF can’t get that small without making its pictures hard to read (never below 72 dpi at quality 30), you get the smallest copy we could make, and the row says so and shows its size. The pages for the limits people ask for most are Compress PDF to 100 KB, 200 KB, 500 KB, 1 MB and 2 MB; the other sizes are listed further down this page and on each of them.

A PDF protected with a password isn’t compressed, and neither is one with a digital signature: changing it would take the protection off or break the signature, so it is left exactly as you added it, and the row says why.

Some pictures stay as they are too, so their colours don’t shift: CMYK photos made for print, scans saved as black and white (CCITT and JBIG2), JPEG 2000 pictures, and pictures with a stencil or colour-key mask. A PDF that can’t get smaller comes back as the file you added, marked as already as small as we can make it, and never as a bigger one.

The PDF is read from your disk in pieces and never leaves your device, so there is no queue and no size limit set by a server. The limit is your device’s memory: computers take PDFs up to 1 GB, phones and tablets up to 300 MB, and iPhones and iPads up to 200 MB. A picture with more pixels than the device can safely handle (a few dozen megapixels on a computer, fewer on a phone) is left as it is rather than risk the tab running out of memory.

Everything runs in a worker, so the page stays usable while a big file is working, and cancelling stops it at once.