Learn Dutch: Ieder type document is hier verplicht. - Every type of document is mandatory here.

Because the word it modifies, type, is a het-word (het type). When distributive determiners like ieder or elk come before a singular het-word, they do not get an -e ending. If it were a de-word, it would be iedere.

Could I say Elk type document instead of Ieder type document?

Yes! Ieder and elk both mean "every" or "each" and are almost completely interchangeable. Elk is slightly more common in everyday spoken Dutch, but both are perfectly fine to use here.

Why isn't there a word for "of" between type and document?

In English, we say "type of document", but Dutch generally places the two nouns right next to each other without translating "of". You will also see this with words like soort (sort/kind): een soort auto means "a kind of car".

Is verplicht a verb or an adjective in this sentence?

It is acting as an adjective here, meaning "mandatory" or "required". It comes from the past participle of the verb verplichten (to oblige / to require), but in this context it is simply used to describe the document.