Why? - .pdf files need "speeding up" (testing) too..
Is there anything (other than the catch in runtest.php) that would prevent webPageTest from testing PDFs ?
Because the pdf files are not the web pages, maybe? The point is, it's just a part of a page's content, whatever it's the very one object there or not. For a purpose of inspecting the average upload speed of the pages consist of only that file, you ought using pdf editing software like this, as an example
https://edit-pdf.pdffiller.com it also shows the time different browsers require to get an output of it, but the feature is paid