Of course - that's the reason:
- PDF ≠ PDF!
- PDF + EREADER = PROBLEM ;-)
Scanned PDF's are in most cases connected with problem on Ereaders. Scanned PDF's are a couple of pictures (1 picture per page) and in some/most cases text on/above the picture. This text is unvisible. In most cases the text is produced with OCR-software (text recognition) which doesn't function well in all cases. That leads to lots of errors in the text (e.g. instead of 'mouse' let's say 'nroII$e').
This kind of text could produce your error in the searching-function.
Solution:
Do not use this or any PDF.
A.