For a process like this, you’ve got a couple different options, but all of them are going to involve a scheduled workflow that runs on a daily basis.  The basic premise is that your workflow will run, search for all of the relevant documents, stash them in a multi-value token, and then send an email out that includes that token (and therefore, a list of the documents).  The specifics are up to you.  Maybe you can use ‘Search Repository’ to find all of the documents, which will make things a little easier.  If you can’t use Search Repository, you’ll probably want to have another workflow that gets invoked every time a document is flagged as completed, and writes the name and/or entry ID of that document to a database.  Then your scheduled workflow will query that database, grab all of the entries, send out the email, and then clear out the database so you’ve got a clean slate for tomorrow.