If the source files can be fixed so that all elements are 100K, that would of course be best. Then the PDF would be correct from the get-go...
If one only has Adobe apps and no other Acrobat plugs, then I would use one of two "hacks" to get the job done. If you have to many of these jobs, it would be of course best to get the job done correctly up front.
---
OPTION 1 (Don't crucify me, I know all the general warnings against doing this!):
In Acrobat, go to Doc props and the fonts tab and write down the fonts used. Load said fonts. Open the PDF into Illustrator, replace fonts as needed with local versions and recolour vector elements to be 100K. Placed images may need to be extracted and fixed in Photoshop, then re-linked in Illustrator.
---
OPTION 2:
Convert a dupe of the original PDF into grayscale mode in Acrobat. Place the PDF into InDesign. Copy/Paste in Place the PDF over the top of itself. Select Colour Burn as the blending mode of the top dupe. Export out a new PDF. Convert to grayscale in Acrobat if needed (preserve black and or preserve cmyk primaries option on). This of course means that the PDF will have transparency used, which would need flattening in the file or at print in a PostScript workflow.
An example of this "Colour Burn Blend" hack is attached.
Best,
Stephen Marsh