Linked PDFs do not open in AIR help window

I am creating a help system in RoboHelp 10 and outputting it as AIR help. The help topics include links to several PDF documents, which are stored locally (they are not on a server or Web site). When I click any of the links in the AIR help window, however, the window goes blank; the content from the PDF does not appear. No other windows open, either. How can I get the content from the linked PDFs to appear in the AIR help window? Thanks!