There isn't any "best" PDF reader, you must to test all of them and install which you consider better.

Are there more than one? I can only find one in the marketplace

FxOS can read pdfs by default (pdf.js) - you can check this by opening a pdf from the internet. Because of this, you just really need an app to browse to the file. I like 'File Manager', but there are others available.

But if I switch away to another app. The PDF is closed in these caaws