paper knife vs letter opener

paper knife and letter opener both are nouns.

Word	Noun	Adjective	Verb	Adverb
paper knife	Yes	No	No	No
letter opener	Yes	No	No	No

As nouns, paper knife and letter opener are synonyms defined as:

paper knife and letter opener: dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books

Other synonyms of paper knife include paperknife.

paper knife (noun)	letter opener (noun)
dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books	dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books