I saw “upon” in a document where “on” would be correct.

Problem:
“Upon” and “on” are not synonyms.

Explanation:
The primary definition of the preposition “upon” is up and on — conceptually, a contraction of the adverb up and the preposition on. A typical final definition indicates that “upon” can be used in place of “on”; however, this substitution is only preferred for metrical or euphonic reasons.

Solution:
Use “upon” when you want to indicate up and on; use “on” otherwise.