Here's another way to do it:
This method requires a bit of initial set up but once you have everything you need ready, its pretty simple.
Step 1: Set up a dropbox account. It is free and easy and super useful. Your students will thank you for teaching them how to use it. You should also install the dropbox app on your iPad.
Step 2: Set up a sendtodropbox account. This is free and simple. You will then have an email address to which you can send files and have them added to your Dropbox. Many apps don't require this step, they will allow you to upload to your Dropbox directly. However, some apps don't play as nicely with Dropbox (i.e. Pages) and you will need this eventually.
Step 3: Copy the link. In the Dropbox app you want to select your document and then tap the link button (it looks like a piece of chain). Then choose "Copy Link to Clipboard."
Step 4: Paste the link into your Google Form. This step requires that you have already created a Google Form and put it somewhere that students can access it. I like to put my Google Forms on my Google Site.
Step 5: Open your Google Spreadsheet to access the student link and record comments and/or score.
I know there must be an easier way to do this. But this is the best I've got for now.
Any suggestions and/or feedback is appreciated!