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PeekPlain.dmg is on its way to your Downloads folder. Here is how to install it, about a minute in all.

Download didn't start? Download PeekPlainmacOS 15 or later. Apple silicon and Intel. Free beta.

Install in four steps

Open PeekPlain.dmg from your Downloads folder (or click it in Safari's downloads list).
Drag PeekPlain into Applications in the window that opens, then eject the disk image.
Open PeekPlain from Applications. The beta is not notarized by Apple yet, so the first time macOS stops it with "“PeekPlain” Not Opened". Click Done, open System Settings ▸ Privacy & Security, scroll down and click Open Anyway next to PeekPlain, then confirm. You do this once for each download.
Pick what PeekPlain opens. On the first screen, choose PDFs, images, Markdown or more, and click Open my files with PeekPlain. You can switch back to Preview any time in Settings.

Why the extra step? Apple's notarization is a malware check for apps downloaded from the web. PeekPlain will be notarized before version 1.0; until then macOS asks you to confirm once. The app works offline and sends nothing anywhere; see the privacy page.

Updates

During the beta, download again from this page to get the newest build; it replaces the old one when you drag it into Applications. Until the app is notarized, macOS asks you to confirm each new build once (step 3). Automatic updates arrive with notarization. The changelog lists what changed.

Removing it

Drag PeekPlain from Applications to the Trash. If you made it your default viewer, first open PeekPlain ▸ Settings ▸ Default Apps and click Restore Previous Defaults.