The features available for Container fields changed with FileMaker 12. Are you using FileMaker 12 or an older version?
If 12, are you using Mac or Windows?
What exactly are you doing to get "a document preview"?
Were the PDF inserted with Insert File (pre 12 windows), Insert picture (pre 12 Mac) or Insert PDF (Interactive portals in v12)?
There are several ways to open a PDF in your system's PDF viewer, but the exact details will depend on the version, platform and whether the PDF was inserted with the "store a reference" option or not.