"Is it woolen?"
Would it be acceptable to say αυτός είναι μάλλινος? or αυτή είναι μάλλινη? Or in this case, the word woolen is used as a noun?
As a translation of the above sentence it is not acceptable because it is αυτό, not αυτός or αυτή. Μάλλινος is an adjective though, and if it characterizes a masculine/feminine noun, it becomes as you wrote above. This sleeping bag is woolen=Αυτός ο υπνόσακος είναι μάλλινος. This blanket is woolen=Αυτή η κουβέρτα είναι μάλλινη.
Thanks for the answer! Your reply actually led to another long time confusion of mine. The "it" in English could be αυτός, αυτή, αυτό, but when referring to something unknown in nature, for example a creature you see in the woods at night, or something you didn't quite get a good look at, and you just want to refer to it as "it" (a thing), rather than he/she (because you don't know what it is and what the gender is the noun), do you always use αυτό for this purpose?
ShaoxuanLi, to answer your original question, my answer is yes! Without context, the pronoun "it" can refer to an article of any gender; therefore, it can be "αυτός", "αυτή", "αυτό" and "μάλλινος", "μάλλινη", "μάλλινο". As a side note, unless you want to emphasize the subject represented by the pronoun "it", refrain from translating the pronoun in your Greek sentence! "αυτός είναι μάλλινος;" means "Is THIS woolen?" as opposed to "Is it woolen?"