Chrome suddenly automatically opens PDF files in Adobe Acrobat after download

This is not an Acrobat problem. Firefox 17 handles PDF files natively without using a plugin and by default PDF files now open in the Firefox PDF preview window.

To change this, go to menu Tools -> Options, click on 'Applications' and scroll down to "Portable Document Format (PDF)" (that begins with 'P', so go way past the 'Acrobat ' files at the top)).

To the right you have some options. By default it is set to 'Preview in Firefox'. Change this to something else; 'Always ask' or, if you prefer the Acrobat plugin 'Use Acrobat Reader', choose the Acrobat plugin option (this requires Acrobat Reader to be installed; if Foxit Reader is installed it will instead show Use Foxit Reader, Best Viewer for Ev...)

FYI for others: if you're okay with the Chrome PDF viewer, but want only downloaded PDFs to open in your external PDF viewer (Adobe Acrobat, Foxit Reader, etc.), you don't need to disable the Chrome PDF Viewer.

Always open PDFs using your system's viewer

Simply click the arrow next to a PDF download and select the option to use the system viewer.

Turn off auto-open of PDFs in the system viewer

UPDATE 6/7/2017

As of Chrome version 59, the option below doesn't exist in Settings. I'll leave these instructions, because if they ever add it back it'll probably be in the same place.

1. Click on the Chrome Menu icon
2. Click on
3. Scroll down and click
4. Scroll down to Downloads. If you have enabled any file types to open using the system viewer, you'll see a button to Clear auto-opening settings. Clicking this will reset the settings for all file types.

More info

Relevant SuperUser discussion here: How to automatically open "saved" pdfs in system viewer on chrome?