Compress a PDF on mobile

Nothing to install

Open the compress tool in Safari or Chrome on the phone, tap the file picker, choose a PDF from Files, iCloud Drive, Google Drive or wherever it lives, type the size you need and tap the button. The result downloads to the phone like any other file.

The reason it works without an app is that the whole engine is JavaScript and WebAssembly served to the page. Your document is read by code running in the tab. It is never sent anywhere, which also means it works on a phone with a bad signal or none at all once the page has loaded.

The measured phone ceiling

Phone browsers kill tabs that use too much memory, and iOS does it without warning. The measurement was taken on an iPhone 16 Pro class device, 8 GB of RAM, iOS 26.6, Safari 26.6: the tab died at a hard 2 GiB per-process limit, with 494 MB of system memory still free at the moment of the kill.

Two things follow. The cap is a fixed budget for the tab, not a reaction to what else the phone is doing, so closing other apps does not buy headroom. And the figure belongs to that device class. A 4 GB phone gets a smaller budget, and nothing here measures how much smaller.

The same probe found a page-level limit: a canvas tops out at 8192 by 8192 pixels. The largest page in the test corpus at 150 dpi is 9.9 million pixels, which is 15% of that. So a single page is not the thing that breaks a phone.

How many pages a phone gets through

The thing worth knowing is how many pages a tab survives in a row, and that was measured too. A 300 page run at 150 dpi finished every page in one tab with no kill, at a median of 671 ms a page, with the per-page cost drifting up 17% across the whole run. The tab does give the memory back between pages.

That is one device and 300 pages. A longer document on a smaller phone is untested. If you have a several hundred page scan and a four year old handset, do it on a laptop.

Speed is the other difference. That phone encodes a 12 megapixel page in about 662 ms and has four cores. A laptop finishes a whole ordinary document in about the time a phone spends on a handful of pages, so a long file is worth moving to a bigger machine even when the phone would survive it.

Where the file goes afterwards

The compressed file lands in the phone downloads, which on iOS means the Downloads folder in Files and on Android the Downloads folder of the device. From there you attach it, share it or move it to a cloud folder the way you would any other file.

Keep the original until you have looked at the result. Compression is a one way trip. The detail the lossy methods drop is gone from the output, and the only way back is to compress the source again at a larger target.

Phone scans are the common case

Most PDFs that are too big on a phone got made on that phone: photographed pages, saved straight to PDF at camera resolution, ten of them, 40 MB. That is the easy case for compression, because scans met their size target on 88.9% of the public bench cases while typed text documents managed 20.0%.

Getting the scan right in the first place saves the compression step entirely. The phone scanning guide covers resolution, lighting and page detection. The general compression guide explains what is making a given file heavy, and the compress hub lists the rest of the cluster.