Official documents should be signed with black pen or black ink or blue-black ink. Ballpoint pens can also be used for unimportant documents, but they are generally not recommended.

It is common sense that pencils can't be signed. If someone really doesn't know, you can remind them in person, and there should be no objection.

Of course, if the sender of the document is more careful, it is best to put a signature pen in the document.