You can see idf file in any text editor. AS TEXT. It is used as open transfer file from PDMS to some special prpgramms for calculating for example pipe electrical heatiing and so on. If you need to open these files for checking use text editor.
As Cody says SmartPlant Isometrics is not free. With most things that are free you get what you pay for, this is not the same situation as something being included in the price though :)
If you look at some of the links on the product web page though you can get some understanding of the value of the software and its function.
It would help to better understand what Sebaztian wants to do with these IDF files once he can see them? As build them, tie into them, import them into pdms? Hopefully he can explain the problem a bit further.
I agree with Doug68, we need a bit more information.
Just FYI, AVEVA does have an equivalent to SP Isometrics called AVEVA ISOMET [url=http://www.aveva.com/Products_and_Services/Product_Finder/AVEVA_ISOMET.aspx]AVEVA | AVEVA ISOMET [/url]