letter opener vs paper knife

letter opener and paper knife both are nouns.

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

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

letter opener and paper knife: 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 paperknife.

letter opener (noun)	paper knife (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