>We would like to post purchase orders converted to PDF files for viewing/printing by our vendors via our web site. However, how do I protect these files for access by only those "applicable" vendors for each P.O.?
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>I guess my question is how do you dynamically allow/restrict access to specific files on the web server. TIA.

Here's one way: Put the PDF file names in a database along with the vendor ID for each one. Require the vendors to login to your site. Once you have validated the vendor login and know who it is, you can dynamically generate a page that contains links only to the PDF files that belong to that particular vendor.