PKCS#7 signed data structure in signed PDF document
The actual data can be omitted from a SignedData encapContentInfo; see current CMS RFC which is not materially changed from earlier versions. The RFC calls this "external signature"; IME it is also called "detached signature" or "clear-sign[ing,ed]", and is widely used. For example S/MIME signed messages usually are a multipart containing the plaintext data and separately (the encoding of) a no-data/external/detached signature on that plaintext; this allows the text part to be handled by a non-SMIME-aware or even non-MIME-aware program (only without signature verification).