Thank you for your post.
For security reasons, iOS places each application in a "sandbox". A sandbox is a set of controls that limit the applications access to files, preferences, data, etc. As part of this process, iOS installs each application in its own sandbox directory, which acts as the home for that application and its data. Therefore, in FileMaker Go, when you store a file in a Container field, it is available only in that application. When you select the file to open in "PDF expert", you switch to the sandbox of "PDF expert", where it will then open, and any changes will be saved to the sandbox of "PDF expert".
Let me know if you need additional clarification.