PDF Reader doesn't work

when I open the pdf file, the PDF reader doesn’t work, but the download progress works fine. Kindly someone here can help me.

I have a similar problem. When I open a pdf file, it’s the Ebook (epub) reader that opens and not the regular PDF viewer. It’s annoying because it is slow to load. What should I do to stop pdf files from opening from the epub reader? Hope to get a response soon.