How to open a .pub file without Microsoft Publisher
Someone emailed you a .pub file. You're on a Mac, a Chromebook, or a Windows machine that just doesn't have Publisher installed, and it won't open. Here's why, and what to do about it — in the order that actually helps.
Why it won't open
.pub is Microsoft's own proprietary format, and — unlike Word, Excel or PowerPoint — Publisher has never had a free viewer. Microsoft's own support page says it plainly: "Publisher doesn't have a viewer for opening and viewing Publisher (.pub) files." (support.microsoft.com) There's no lightweight app, no browser extension, nothing free from Microsoft that just shows you the page. This isn't new because of the October deadline — it's always been true. It's just about to matter to a lot more people at once.
If it's one file: ask for a PDF
If it's a single file, or a handful, the free and fastest answer is to ask whoever sent it to export it as a PDF. Anyone with Publisher can do that in under a minute — File > Save As > PDF — and a PDF keeps the layout the way its author saw it, opens on literally anything, and costs nothing. Ask before you try anything else.
If the sender is gone, or nobody involved has Publisher anymore, LibreOffice Draw opens.pub files for free — Windows, Mac and Linux — through the same open-source parser this site uses, converting one file at a time entirely on your own machine; nothing gets uploaded anywhere. Simple, text-heavy files tend to come through fine. Complex layouts, lots of images, or unusual fonts tend to arrive scrambled, with pictures shifted or dropped. (ask.libreoffice.org) Worth trying before paying anyone for one file.
A note for Mac and Chromebook users specifically
Microsoft's own bulk conversion script, Convert-PubFileToPDF.ps1, needs a licensed copy of Publisher installed on Windows — it drives Publisher through COM automation rather than reading the file itself, and it has never run on a Mac or a Chromebook. (download.microsoft.com) That's true whether it's one file or a thousand: there is no official Microsoft path that runs on either machine.
More files than you want to open by hand
If this isn't one file but a real pile of them, see how to batch convert Publisher files for the free options and where each one breaks, or use the converter on this site to handle up to 200 files at once in a browser — Mac and Chromebook included — with a report on which files need a second look before you trust them.