If you are storing PDF documents in container fields and you are using at least FileMaker 12, you can read the PDF right in the container field. You even open a window with a single button click to a layout where the container field fills most of the screen for easier reading and close the window when you are done. You would optimize the field for interactive content to make it possible to read each of the pages of the PDF. Interactive container fields do not work inside a portal, so you have to use a field not located inside your portal row for this purpose.
Other document types would need to be exported/opened in many configurations but this can be scripted to take place with a single button click and you can export the copy to the temporary folder on your computer and then not have to be concerned with multiple copies of your files cluttering up the computer as these exported copies will not persist.
If, on the other hand, you insert them as files with the "store a reference" option into container fields that are not set up for external storage from a properly shared directory, you can indeed double click the container field (if it is not optimized for interactive viewing) to open the original copy of the file in your system's default application.
What Phil said. I am using FileMaker Pro 13 to manage scanned PDFs. The Adobe reader shows on the container field to print the page or pages we need. Has worked very well. As I am still trying this, as is my department with regards to scanning the receiving documents, I am using to import the PDFs and not the reference. The permanent location has yet to be determined.