Question: so with this, I can send an attachment and change the file before the recipient opens it? Will they see if it has been modified? Will I see when they have accessed it?
This is an automated way to do what was previously "copy and paste a Google Docs link into an e-mail".
Yes, you can change the file. If it's in native Google format (e.g. a Google Doc, not an uploaded/stored Word .doc file) you have a revision history. You can see when people join/leave the document while it's open (without timestamps), but you can't bring that up later.
