Thank you for your post.
At this time, FileMaker Go cannot access files outside its own work area. If you can first move the PDF file into the FileMaker Go work area (through iTunes), then you can insert the PDF file into the Container field.
I recommend you enter this as a suggestion into our Feature Requests web form at:
The entries into this web form populate a database hosted by Product Management and Development. All suggestions area reviewed and considered for future releases. Although I could copy your post and paste it into the web form, there are a couple of questions asked that only you can answer.