Although I don't have the volume of words as the OP, I have the same general issue: I use a typewriter for drafting, and scan the drafts as backup. There's a number of us who do exactly this for NaNoWriMo each year, making an already sanity-limited exercise even more complicated.
There are a variety of packages out there:
OmniPage always gets high marks from my fellow typists whenever this topic comes up. I haven't tried it myself, though. Both Word and the full version of the Adobe software have an OCR engine built in or installable, although it may take some digging around on searches to find it.
As was stated above, the PDF is really a "wrapper" around the actual image file itself. I don't know if OCR software is smart enough to undo the wrapper. Adobe might be your best bet, or even some image software like
Gimp, which can open image-bearing PDF files, and can re-save into a variety of formats, including ones that your OCR program is bound to like. Images saved as TIFF seem to be universally accepted by OCR programs, being a format that scanners also tend to use.
No matter what, though, you're going to fall prey to the GIGO principle: garbage in, garbage out. If your typescript is at all smudged, or if the scans were of low quality, then it's very possible your recognition will be more in the 50%-or-lower range. If the typescript was prepared on an electric machine with a carbon or correctable ribbon, then the impressions should be clear and consistent. If however, you used a manual machine with a standard ink ribbon -- as I do -- your letters may vary in darkness and clarity, and it's that sort of inconsistency that gives OCR programs fits. Even having to correct every tenth or fiftieth character gets tedious very quickly.
In the past, I have retyped my mss. into the computer: the typed page is considered a draft, I draw all over it in pen, and then type in the corrections. I've got one waiting for me that I'm going to try to read into the computer with
Dragon Dictate software when I can get some time to sit down and really use it. OCR simply doesn't work for me with the state of my drafts and my terrible typing technique.