You need to put the focus on/in the container field. That can be done by first clicking the container field, then clicking your button. You can also use your button to run a script so that it can do more than one thing. If it ran a script like this:
Go to field [yourtable::yourcontainerfield]
InsertFile or InsertPicture or InsertQuickTime.... //pick one that works best for you
You can just click the button and the insert opteration should take place.
One more question, is it possible to upload many pdf files in separated containers?
Yes, but you'll need to figure out how to tell FileMaker what files to insert and in which record/Field. You may want to acquire a plug in that llows you to read the files in a directory so that a script can loop through such a list, inserting each file in a different container. Usually, you'll want to use a related table of records with one container field in each record for this.
Plus, I was searching about the super container, is it safe to download (How to use it as well)? Does it store my pdf documents in its server?!!
I've seen enough experienced Developers recommend this product to be pretty confident that it's "safe", but expensive. The best soure of answers to questions on how it works and how to use it is to visit the developer's web site and ask them directly.
Thanks Phil for your help :)