FOUND A SOLUTION:
1. Copy all your old pages or numbers documents that are in iCloud onto your computer. If you don't have a computer, you can email all of the documents to yourself so that you have them stored in a safe place.
2. Close all your apps.
3. Open the offending app [pages or numbers], then delete all your documents from iCloud (I did this on my iPhone)
4. Quit pages/numbers
After doing this, my phone no longer restarts upon exiting Pages.
I haven't tried re-uploading any of the deleted documents to iCloud, because I don't really care to have my old documents in iCloud.
I had this idea when I installed numbers on my iPhone but did not have any of the issues described above, however, pages was still crashing my 5S every time.
I still think this solution is unacceptable, and I am very disappointed in Apple for not catching this bug. It's ridiculous that I figured this out before Apple did. Not even my app...