PDF is an undocumented feature. My tests reveal that each page of a PDF is imported as 1 second of video. A 5 page PDF will occupy 5 seconds in the storyline, each page occupying 1 second.
It's an interesting "feature". It has its benefits. But it is oddly handled. You actually have to Retime the clip (command R) to time stretch it, either shorter or longer.
As an end note: the vector graphics do not have infinite resolution in fcpx; they're only as good up to the preview size of the PDF. This can be "managed" somewhat by opening the original PDF in Preview, going to Print; setting the options for the printer [Printer Features] to be a high dot resolution [I have one that's 2880 x 2880 which is a square pixel aspect]; then Save as PDF (don't physically print.) I recommend changing the print size to a larger format than Letter. [All of this might be predicated on your installed printer drivers... I don't think I have any PostScript drivers installed.]