The file names the Reader uses are from the metadata, not the document title. You can find out what that is, by opening it in Acrobat or the Reader, and going to File-->Properties; it'll list the title, author, subject and keywords (if any).
To change it, you'd need Acrobat Professional, or a program like
Change PDF Metadata. (I have no idea how good that is; I use Acro Pro.) This won't work if the PDF is locked or DRM'd.
How it gets that way: the metadata is created by whatever program was used to create the PDF. Some of them assign a nearly-random number; some use the text of whatever the first "heading" style was; if they're created by batch by another program, all sorts of things can happen.