Looks like LimeWire is checking *inside* the file you downloaded to see if it's really a .PDF file and it's not.
I wonder if it saved it anyway and is just warning you?
Could this be a add on program that is doing the checking? Like a virus scanner or something?
It does sound like you downloaded a PDF file that really isn't a PDF file, some people have been renaming files wrong.
Try getting it from another source.