Asking whether the document is PDF or Excel demonstrates a lack of understanding. The document type is irrelevant. It is a file of bytes. You want to send those bytes securely. (And you may want the receiver to be able to verify that it actually came from you.)
It demonstrates either a lack of understanding or more than a passing familiarity with the applications. MS Office has used AES for the built-in encryption since Office 2007. That would seem like a reasonable choice for sending a file to a contact, provided you choose a strong password and communicate it out-of-band.