Adobe Acrobat
I would recommend using Adobe Acrobat (full version) to create a PDF form that contains blank fields that are uniquely identified.
I would then use Progress to generate an FDF text file that contains the name of the blank PDF form, and also the field names and desired values.
Clicking on the FDF text file will first open the blank PDF and then populate the fields with the values from the FDF. This can be done with Adobe Acrobat Reader. The document can then be printed or saved.
Usually, creation of documents like purchase orders need to be automated, and this will require a scripting tool and server process. Let me know if you would like some ideas or assistance on how to develop an automated process. Sending the FDF file via email can also be automated but would require the master PDF document to be placed on your website.
One big advantage of using the FDF technique is the size of the file is much smaller than a document in PDF format.
This is a good tutorial on the topic:
http://www.planetpdf.com/mainpage.asp?WebPageID=338