Learn French: Faire le tri parmi ces documents est difficile, je le sais. - Doing the sorting among these documents is difficult, I know it.

We use parmi because we are talking about a general collection of items. Parmi translates to 'among' and is used for a larger, undivided group. If you were sorting between two specific, distinct things, you would use entre (between).

What is the difference between faire le tri and simply using the verb trier?

Faire le tri is a common set phrase meaning 'to do the sorting' or 'to separate things out'. You could perfectly well say Trier ces documents, but using faire le tri emphasizes the whole process or chore of sorting through a messy pile.

What exactly does the le in je le sais refer to?

The pronoun le here is acting as a neuter pronoun. Rather than replacing a specific masculine noun, it replaces the entire preceding fact: that sorting the documents is difficult. While English speakers often just say 'I know', French requires this le to represent the known idea.

Can the verb Faire just sit at the beginning of the sentence as the subject?

Yes. In English, we usually use the '-ing' form for this, like 'Doing the sorting'. However, when a verb acts as a noun and the subject of a sentence in French, you must always use the infinitive form (Faire), never the present participle (Faisant).