Open A Pdf By Entering Reference

We're in the process of re-writing the Intranet at work, and my javascript
knowledge could be written on the back of a post card. ;-)

Can any of you point me in the direction of some code that would enable a
form to open a specific pdf file? Details are below.

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All of our purchase orders are saved back to pdf files. Each pdf has the
prefix "PO_" followed by the po reference, then the file extension ".pdf".
For example, our file would be called "PO_12345M.pdf".
Each set of files is archived into years (folders are called 2003, 2004,
2005 etc.) so it will probably need a drop down box to select year, unless
it can search through all the folders.

The visitor should only need to enter the PO reference, and the code puts
the "PO_" and the ".pdf" on the end of it, and hey presto Acrobat opens with
the PO, or you get an error dialog if the file cannot be found.
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