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By PDF I assume you mean PDIF. You can't directly import PDIF files into Protel or AD. You need to open the PDIF file in another format and then import into AD. The easiest is PCAD. Open the PDIF file in PCAD, then save the PCAD file in ASC format. Now you can import the PCAD ASCII file directly into AD.
If you have trouble, PM the file to me and I'll convert it to Protel/AD for you.
PDIF is an ASCII file format used by PCAD and several other old PCB programs. It also uses the file extension "PDF", but it isn't the same as a document PDF. If you're sure your file is a PDF document file, and not a PDIF PCB file, you'll have to use one of the above techniques to get it into PCB format.