You can use Insert File with the "store as reference" option to insert PDF's into your container field. You can script this and set your field up as a button to run this script when it is clicked if you want.
There's some pretty good starting info on container fields in FileMaker help. Have you searched help for info on Container fields?
When I get this to work, I intend to also use it at the national level with over 250000 records.
You'll definitely need to research your options carefully here. If this is a database you intend to share over a network or publish to the web, "by reference" container field behavior changes. The files must be located in specific places that are accessible to your users. On a local area network, this would be a shared (can be read only) directory to which your users have direct access via the same filepath stored in the container field. In mixed platform (mac and windows) networks, this can be a bit of a challenge to deal with. If you publish to the web, the files must be stored in the correct location in the web folder. See the PDF file that comes with fileMaker (see product documentation in your help menu) on publishing to the web for more detail.
And if any users will be using an iPad or iPhone to access your database, "by reference" images are not accessible to them.
You may want to research a third party add on called "SuperContainer" when you get ready to "go national" with your database.
Thank you, I was assuming that you needed to type the link into the container field. Thanks for setting me straight.