So what you need to do is
1) Download and install PDF995 (or some other pdf creator). Note with PDF995 you need to download 2 files.
2) Open the dtp file using Greenstreet Publisher
3) Within Greenstreet Publisher select the "Print" option (I assume there is one)
4) When the print window displays it will give you a list of devices you can print to. If you have a printer it will be on the list.
5) Open the list of devices and "PDF995" should be on the list. So PDF995 "pretends" to be a printer.
6) Select PDF995 and it should appear in the window. Here is a picture of what it should look like, although in this picture it says "solid print pdf" and in your case it will say "pdf995".
http://www.soliddocuments.com/images/SPprintdi alog.png
7) Continue and eventually it should create a PDF file.
Good luck.