Oh I completely agree with the better performance in general of DWF vs PDF. PDFAttach is more of a 'checkbox' feature than as useful as it could be IMHO. For vector CAD generated files, DWF is great. PDF incorporates so many mixes, that it's often a mess to work with.
For the OP though, given that he's dealing with a 65 year old drawing, (blue-line? blue-print? ink on linen?
) run through a scanner and saved to PDF, I'd expect a lot of pixels, many irrelevant from the background -- probably colored by yellowing of the media, and faded linework that's hard to work with. IronGall ink does tend to brown out with age and oxidation. Dunno about squid ink anymore. Before my time donchaknow.
Heck the 20 year old bluelines I've had to deal with have been messy enough after scanning.