letter opener vs paperknife

letter opener and paperknife both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter opener	Yes	No	No	No
paperknife	Yes	No	No	No

As nouns, letter opener and paperknife are synonyms defined as:

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

Other synonyms of letter opener include paper knife.

letter opener (noun)	paperknife (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