First, it can be done. PDF files can be pasted into the AutoCAD world by using the "snapshot" command in Adobe Acrobat reader. This takes a picture of the PDF that you can then post into AutoCAD using the "paste" command. You can then set the size for the document yourself. This method places the PDF into CAD as an OLE object, which is another type of ActiveX object.
Second, I strongly caution you about posting PDF's to AutoCAD. They can be very problematic. I have done this a few times and have given up on it, as the PDF's often become deactivated and no longer appear or print. You end up with a blank frame outlining a blank image that displays nothing and gives an error message when you try to activate it again. The file is still attached to the CAD file, though, as the file size (in Kb.) indicates that the image files still remain, but they are useless.
I'm not sure if you can insert PDF's using the "image" command. Just type "image", and enter the relvant file type, file name, and other parameters when prompted to do so. I'm going to try this Monday morning to see if it does work.