The GPGServices service plugin adds to the global Services menu (in the Application menu) the facility to sign and verify plain text in any application that supports text services. The plugin also provides convenience facilities to import an ascii-armoured key from text in any application as well as return your default key, in ascii armoured format, and the keys fingerprint.
Signing and pasting your key or fingerprint require the host application to support pasting of text (so you will not be able to sign text highlighted in a pdf file in Preview, for instance). Verifying a signature and importing a key only require that you can highlight the text.
These details are provided for information only. No information here is legal advice and should not be used as such.