Learn Dutch: Weet u of deze folder beschikbaar is? - Do you know if this brochure is available?

You are right that of usually means "or". However, when you use it to ask an indirect yes/no question like this one, of translates to "if" or "whether".

Why is the verb is at the very end of the sentence?

The word of introduces a subclause (a dependent part of the sentence). In Dutch subclauses, the verb is always pushed to the end. So instead of the normal question order is deze folder beschikbaar, the word of changes it to of deze folder beschikbaar is.

Does folder mean the same thing in Dutch as it does in English?

Not exactly! While it can refer to a digital file folder on a computer, a physical folder in Dutch is usually a leaflet, brochure, or advertising pamphlet you would pick up in a store or tourist office.

Why do we use deze instead of dit for "this brochure"?

We use deze because folder is a de-word (de folder). Remember that you use deze for de-words and dit for het-words when pointing out "this" specific thing.