Document
noun
If you put writing into a category, you'd have to write it down after you've chosen what category it is in. Where would you write that down? Into a document, probably.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
それは何の書類ですか？
What is the document for?
この書類、コピーしておいてもらえるかな？
Could you please photocopy this document for me?
大事な書類なので、ゼッタイに無くさないでくださいね。
Please do not lose this important document.