The drive was mapped, but was it mounted? There seems to be a difference. Try opening a folder on the mapped drive, then try your save as PDF script.
The error message you get when this step fails is frustrating as it really doesn't tell you enough to know how the save failed only that it failed and what file name it was using when it tried to save the file. Often, little picky details not quite right in the file path will derail the save until you figure out where it went wrong for the save as PDF step.
You may want to post the exact script steps you used so others can see the calculation used to produce the file path and file name. Sometimes others can see what we can't.