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.

Unterlagen is a common word for documents/paperwork/records—the files or papers you keep for reference (e.g., invoices, receipts, forms). It’s usually plural in German.

In the dative plural, German very often adds -n to the noun (if it doesn’t already end in -n/-s).
So: die Unterlagen → den Unterlagen.

Both are correct. Putting Den Bon first is topicalization: it moves the object into position 1 to emphasize it or connect it to the previous context. The finite verb still stays in position 2 in a main clause:

• Den Bon (position 1) hefte (position 2) ich ...

Not really. When something other than the subject comes first, the subject often comes after the verb:

• Den Bon hefte ich ...
  This is normal German V2 word order.

damit introduces a purpose clause: so that / in order that.
It creates a subordinate clause, so the conjugated verb goes to the end: ... finde.

Because damit starts a subordinate clause, and subordinate clauses in German typically place the finite verb at the end:
damit ich ihn später finde.

ihn refers back to den Bon. Since Bon is masculine, the accusative pronoun is ihn (him/it).
It’s accusative because finden takes a direct object: etw. finden.

Yes. Repeating the noun is possible, but using the pronoun ihn is more natural because den Bon was just mentioned.

No. später is flexible. Common options include:

• damit ich ihn später finde (very natural)
• damit ich später ihn finde (possible, but a bit less smooth)
• damit ich ihn finde, wenn ich ihn später brauche (expanded idea)
  In general, adverbs like später often go in the middle field before the final verb.

They overlap but aren’t identical:

• der Bon: typically a receipt from a shop (often casual/everyday).
• der Beleg: more general, can be receipt/voucher/proof/document (often more formal/accounting).
• die Quittung: a receipt/acknowledgement of payment, often when someone explicitly “issues a receipt.”
  In this sentence, Bon fits well for a store receipt.

Yes, because the subject is the same (ich) in both clauses. You can say:
Den Bon hefte ich zu den Unterlagen, um ihn später zu finden.
Difference:

• damit + finite verb is more general and works even when subjects differ.
• um ... zu + infinitive is common when the subject stays the same.

German uses a comma to separate the main clause from a subordinate clause introduced by words like damit, weil, dass, etc.:
..., damit ich ihn später finde.

It’s idiomatic and understandable. Depending on context, you might also hear:

• Den Bon hefte ich an die Unterlagen. (an
  • accusative for attaching onto)
• Den Bon lege ich zu den Unterlagen. (just “I put it with the documents,” no stapling implied)
• Den Bon hefte ich an/zu die Rechnung. (attach it to the invoice, common in bookkeeping)