You will have to check the application that you use to send the PDF file because it's possible that configured settings change the format to DAT. If you still have issues, export the PDF file on your computer, then manually send it to the other computer using network sharing or simply email it.
Additionally, you can use a portable HTTP server like HFS: http://www.rejetto.com/hfs/
Download the EXE, launch it, add the PDF file and share the IP.