Mass nouns, such as "soap" or "fruit" do not appear in the plural when they are used in their non-counted, non-discrete sense.
You can't use a number adjective in front of words like this, so they don't form the plural. You can't say "I need five papers," for example. You have to say, "I need five pieces of paper."
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"PaperS" in the plural usually means something different from "paper."
"Papers" refers to newspapers, or to other written articles (student assignments, perhaps, or scholarly articles published in professional journals, etc)
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