Learn Swedish: Vi hänvisar till dokumentet som kräver din underskrift. - We refer to the document that requires your signature.

You would use härmed. It typically goes immediately after the verb in a main clause, making the sentence: Vi hänvisar härmed till dokumentet... (We hereby refer to the document...).

Why use underskrift for "signature" instead of signatur?

In Swedish, underskrift specifically means a handwritten signature at the bottom of an official document or contract (literally "under-writing"). Signatur is used more for an artist's signature on a painting, an email signature block, or a short initialing.

Does hänvisar always need till when it means "refer to"?

Yes, when you are directing someone's attention to a document, place, or person, hänvisar till is the standard fixed pairing. Without till, it loses the meaning of referring someone to a specific thing.

Is kräver too aggressive here, since it often translates to "demands"?

Not in this formal context. While kräver can mean "demands", it is the standard, neutral verb used when a rule, document, or system requires something in order to proceed. It doesn't carry the emotional pushiness of a personal demand here.