How to access every field in a document (in a loop)

I want to validate all fields before printing and have a useful example with working script. But when I import a PDF and use it as a fixed background for my forms, I get an error inside this validation script that runs through all the fields by using
...
xfa.layout.pageContent(nPageCount,"field");
...

how can I solve this issue? / Howelse can i loop through all the fields?
And why does everything work when i import as floating stuff? (But that's not possible in my case)

thanks a lot.
fabian