Thanks George; however, I have read this and as I am only new to this, I still can't figure it out.
In order to be able to do it you have to install a script in a local folder of each computer where this file is going to be used.
Is that possible in your case?
No it's a generic form that lots of different people will need to fill in. Was hoping that they would be able to save it in a certain format so that when I receive the document their surname (from TextField) appears at the beginning of the file name.
In that case you won't be able to do what you want. You can create a file path string and place it in oanother text field, which the user can then copy and paste for use with the Save As dialog, but that's about it.
If it's a form lots of people will fill in, have you confirmed none of them will be on iPhone, iPad or Android tablet? That they will be happy and willing to install Acrobat Reader? That they know how to download and open in Reader?
In that case you can't do it. You can suggest to the users to save the file under a specific name, but not enforce it.
However, you can prevent the file from being submitted back to you if it doesn't have the correct name...