The array of objects that you think are NSStrings are actually PFObjects. PDFArray contains an array of PFObject's. You probably want to be grabbing a certain property in the predicate you created in your UISearchBarDelegate method,
-(void)searchBar:(UISearchBar *)searchBar textDidChange:(NSString *)searchText
Something like NSString *s = evaluatedObject[@"PDFName"];
, or something to that effect will make it so that you're actually filtering along the name of the PDF file.