Looking at the following query, you'd expect it to return rows that had 'pdf' in either of the two columns.
However, if 'pdf' exists in 'FileSiteTitle' and 'FileSiteComments' is null, the row is not returned.
SELECT FileSiteTitle, FileSiteComments, Path FROM portal..scope() WHERE "SCOPE" = 'All Sites' AND SiteTitle = '45611' AND ListContentType = 'FileSiteFields' AND ( FileSiteTitle LIKE '%pdf%' OR FileSiteComments LIKE '%pdf%' ) ORDER BY FileSiteTitle DESC
I know this is only mimicing SQL query, but it should try better no?
FileSiteComments IS NULL OR FileSiteComments LIKE '%pdf%' but that just throws a COM Exception, even though the is null predicate is documented here
'FileSiteComments' is a "Multiple lines of text" field if that helps.