My pdf documents are labelled Avast html document on the file details, but are called pdf when referred to by other programs, and open as pdfs to look at.
Right-clicking on the file details in the folder, I opened the "properties" at the bottom of the menu.
Under the General tab, I clicked the "Change" box , and chose Adobe from the drop-down list
All the files in the folder then changed to Adobe Acrobat Document as a label - still not "pdf" as I would have expected [the way they used to be pdf].
I think it probably doesn't matter much either way - it just seems that someone - Microsoft? - has decided to drop the pdf label, and call the documents by your chosen "file opening program".
Another thing that someone couldn't just leave as familiar to the computer user. Hey ho.