Before you even start you should have asked yourself, do you have written permission of the arranger of the Mozart Piano piece to upload it here (A. Burford)? If you are not the arranger (as seems likely) and do not have written permission, then the file will be deleted.
Assuming you did have the right to post it, then first, you would go to the page for the composer: Category:Mozart, Wolfgang Amadeus
Once there you would check to see in the Piano Piece K 33B was there or not, and if necessary create a new page for that work.
Once that is done you can add a PDF file to the work page.
In other words, do not use the "Upload file" link - the warning should have told you that.