This is it:
It was the Mayflower Compact.
This is why:
I would explain what the Mayflower Compact is, but the question already does that.
However, a little extra piece of information on the document can be assumed. The ship that took the same settlers of this question to Plymouth was called the Mayflower. It can be assumed that they named the Mayflower Compact after their ship, and simply added compact as a word to describe their treaty (since one of compact's definition as a noun is a formal agreement, similar to this document).
^ Technically this is totally unnecessary, but perhaps it will do a bit in convincing you of my accuracy. :)
If it's still confusing, then here; take a snippet from an official history book that contains the information thou likely seeks:
