Nami;
I have the same problem (but no solution yet). Here is what I have found:
1) Spotlight will not search the text contents of my searchable pdfs (scanned with a ScanSnap and OCR'd with Acrobat 8.0) even if I define the location, kind, and content using cmd-F.
2) The pdfs are "real" pdfs with searchable text since I can copy and paste the text from the pdf to a text file.
3) Adobe Acrobat 8.0 WILL search my pdfs --- that is, the program will search a folder and return pdfs with the specified search term. ~100 pdfs will take about 10 minnutes.
4) I can find pdfs that are converted html or Word documents.
I suspect that searchable pdfs, which are converted from an image, have the text tagged or stored differently than html and Word documents that are converted to pdfs. Any other ideas.