PDF files are opening in a web browser instead of Adobe Acrobat.
Issue:
Latest versions of chromium-based web browsers such as Google Chrome and Microsoft Edge can now function as a PDF viewer. The benefit of this is that your web browser can open online PDF files from an internet source without having to download a foreign file to your local computer to open it with another application such as Adobe Acrobat Reader.
When one of these applications preforms a major version update, it can set itself as the default application for the file types that it supports. This now includes .pdf files.
Resolution:
Software related issues.