You do not need to open the save file in order to send it to Larian. The game itself is the only program that can open and read the file correctly.
I'm not sure why Adobe is giving an error message, unless they are also using the dxg extension for some other type of file (though Google's first few results only list one company using that extension, and it isn't Adobe... or Larian).
In your email program, once you start a new message, there should be a button or menu option to 'add attachment' or 'attach file', or something like that. At least a few years ago Outlook Express used a paperclip icon for the attachment function. You may be able to drag and drop the file into the email program (or the new message window) from Windows Explorer. If you do not see anything like that poking around, check the help file for your email program.
Once you get into the attachment function, there should be a standard 'open file' type dialog box, where you can browse to the save game folder and select the file you wish to send. Regular save files are named savegame00.dxg, savegame01.dxg, etc, but I'm not sure about the quicksave.
If your email program will not browse to the savegame folder, you can do so in Windows Explorer, and copy the save file to another folder (like c:\temp) or your desktop and attach it to an email from there.
Alternately, you could upload the save file to a site like
RapidShare or
Megaupload, and then email Larian the link to download it.
It would be much easier to get the save file sent off to Larian than re-playing the game to get up to that point again.
D2:ED (Divinity 2 - Ego Draconis) is the original game release.
On the PC there is a digital download add-on, FoV (Flames of Vengeance), which can be installed to update D2:ED and allow you to continue with that character after the events of the original game (or start a new level 35 character at the beginning of the add-on).
DKS (Divinity 2 – The Dragon Knight Saga) is a remastered version of the original D2:ED together with the add-on FoV.