paper knife vs paperknife

paper knife and paperknife both are nouns.

Word	Noun	Adjective	Verb	Adverb
paper knife	Yes	No	No	No
paperknife	Yes	No	No	No

As nouns, paper knife and paperknife are synonyms defined as:

paper knife 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 paper knife include letter opener.

paper knife (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