Computer Help and Support

In the Search bar, type Default apps
Scroll way down the page (below Reset) and click on 'Choose default apps by file type'

This list takes a while to come up, patience.

Then, use the scroll bar on the right to scroll all the way down until you see .pdf

You will probably see Chrome selected for .pdfs. You want to Select it, and then change it back to Acrobat.

Close settings, your icons should all look familiar once again.