in my setup, the sitemap generator always reports the current time as the last-modified timestamp for PDF files. Some details:
(1) The sitemap generator is configured to include PDFs in the sitemap, but not to parse them.
(2) It is set to use the server's response for the last-modified tag.
(3) The server sends the correct HTTP last-modified header, corresponding to the file upload timestamp.
Looks like a bug. Could it be that the sitemap generator doesn't do a HEAD request for files which are not parsed, and then defaults to the current time for the modification tag?