I think there is a little confusion here, the question you *asked was how to sort the recordset, the file extesnion wont effect recordset sorting. Sorting is the order in which the results are returned.
It looks like what you are really looking for is how to Search the database which is completely different kettle of fish.
since all of your files have the extesnion ".pdf" the simplest solution will be to append .pdf to the search string.
another way to go about it will be to use the replace function in your recordset:
SELECT REPLACE(filenameColumn, ".pdf", "") as stripedFileName FROM tblName
Get your search page to a point where it is working when you include the file extesnion in the search criteria, then post a copy of the page here, and I will let you know the modification to make so that it will work without the file extesnion in the search criteria.