Questions & Answers about Den Bon hefte ich zu den Unterlagen, damit ich ihn später finde.
Because Bon is masculine (der Bon) in the dictionary, but here it’s the direct object of heften (to staple/attach), so it takes the accusative case: den Bon.
heften can mean to staple or to fasten/attach (often with a stapler, clip, or by filing).
The particle zu is common with heften to express attaching something to/onto something else: etw. zu etw. heften = to attach something to something.
Because zu is a preposition that always takes the dative.
die Unterlagen (plural) becomes den Unterlagen in the dative.