How can I from within Reader navigate to the next or previous pdf in a Windows folder?
Use the menu File > Open...
Not useful for me. I recall with early Reader there was a pdf management section that allowed instant selection of files on clicking a graphic of the first page. Using file open leaves older files open.
May be that you have used a PDF Portfolio.
Ah! before I retired I was using Acrobat and I had forgotten about Portfolio.
There's no such option in Reader.