
Pick your desired app from the list - ideally Adobe Acrobat - and hit "Set default." This will open a dialog box asking you to pick a default app for.

The other is you may have accidentally changed your computer's default app for opening PDF files yourself. The first one is you may have installed a buggy update - either for Google Chrome or Microsoft Windows - that automatically switched all your PDF documents to Chrome HTML files. There are several potential reasons why this is happening.

All of a sudden, it becomes harder to distinguish PDF files from other saved webpages. Of course, some users may find it frustrating when their downloaded PDF documents that used to have the easy-to-spot Adobe Acrobat thumbnail have inexplicably turned into Chtome HTML files that now brandish the Google Chrome logo. Simply put, to view a Chrome HTML file, you don't need to have Adobe Acrobat Reader - you just need Google Chrome. In fact, according to Adobe, Chrome HTMLs are easier to share through social media and the web in general as it provides various types of users a wider avenue for viewing the content online. In general, a PDF that's been converted into a Chrome HTML document is not an error per se.
