Probably this is a more general iOS platform issue: if you open a PDF file that has a lot of text (as most of them do) and tap anywhere, the keyboard opens. It's hard to get it to hide after that. Scrolling a long distance closes it, but maybe that's not what you want to do.

You can select text on a PDF, but you seldom need to type any text, except in search fields and such. So it would be better if the keyboard didn't open at all when selecting text on the pages themselves.