Arrow keys won't work when opening a PDF in a reference window

This isn’t a major problem, but it is kind of annoying. When I open a pdf in a new reference window, my arrow keys won’t work and I can’t scroll down the page. Page Up/Down keys work, unless I hit one of the arrow keys first. If I do that, I have to click on the document and the Page Up/Down keys start working again, but the arrow keys still won’t work. Any suggestions?