Windows – Why can’t I search a certain PDF in Mac OS X, but I can in Windows using Adobe Reader

I have the PDF "GNU Image Manipulation Program: User Manual"
downloaded to both my Windows PC, and my iMac OS X v.10.9.5.

but it doesn't work at all in my Mac, using neither Preview, nor Firefox browser. Not even my newly-acquired (in a desperate attempt at the end of a long frustrating day of troubleshooting as best i know how!) Skim PDF reader!

Then, I thought, maybe it's the reader; perhaps there's some quirk in this "GNU" PDF, that makes most readers unable to recognize all its text as a fully searchable component? (And maybe Adobe Reader is extra powerful in that regard?)

(I tried then double-checking, to be sure that i could use the search function on my Mac in other PDFs, and yes I can! I searched through every one I tried perfectly, and I tried several, — all except this "GNU" PDF work fine!

Maybe the reason I couldn't make at least my Firefox search in this "GNU" PDF in my MAC, is due to the add-on which i have only for Windows not for Mac: Adobe Acrobat 11.0.8.4 PDF Plug-In For Firefox 11.0.8. It seemed perhaps a more functional app than Preview, or than the app I now have in Firefox on MAC, which is the built-in Firefox PDF viewer('Free Foxit Reader').

But, alas, my search for that Adobe Acrobat add-on showed it also unavailable for Macs.

One thing I did find, though this might be just out of my desperation to find anything at all that might be do-able (since this "GNU" PDF is crucial for me, as I depend upon the GIMP program it pertains to and have had no success in installing the contextual help itself and without being able to search this PDF, I am hopelessly lost!)