Who "owns" the problem is a direct function of how you trying to output PDF from AutoCAD 2005.
If you have Acrobat 9, 8, or 7 (starting at update dot release 7.07) Acrobat's PDFMaker supports PDF creation.
If you have Acrobat X installed its PDFMaker support of AutoCAD starts with AutoCAD 2008 (AutoCAD 2005 is not supported).
So, if you are trying to use PDFMaker you have to have the appropriate release of Acrobat installed.
Not sure about Acrobat 7 but for 8 or 9 you could try an Acrobat "repair" (under the Help drop-down).
If you are using the "internal" process AutoCAD has for PDF output then that process is the issue.
Also, if you are in Windows 7 you have to be using Acrobat X (updated fully) to have Win 7 - Acrobat full compatibility.
Of course that puts you in a sticky wicket if you've AutoCAD 2005 in Win 7.