Re: would a custome made adobe acrobat plugin work in adobe acrobat X pro and XI pro?

I speculate (because I've never used the APIs) that one could have said "Acrobat 9,X,Xi do not provide programmatic APIs to the comparison feature new in 9.0". It seems to me that the Acrobat 8 SDK contains the APIs

DigSigComparePages

DigSigComparePagesEx

DigSigCompareWords

DigSigCompareWordsAndFontsRecent

DigSigCompareWordsEx

DigSigCompareWordsRecent

and according to the documentation for the Acrobat 11 SDK they are still there.

You will notice that the main complaint in the thread was a wish to have programmatic access to the Acrobat 9 compare feature, while the APIs were stuck with version 8; the exact opposite of your wish as you prefer the old.