The das in the first example is not an article since an article would have to come just before a noun. It's a demonstrative pronoun roughly translatable as "that", although "this/these" is more appropriate much of the time. Coincidentally, it has the same spelling and pronunciation as the article das for neuter nouns, but you should regard it as a different word.
In the second example, die is actually the plural article, not the feminine. It's used for all plurals regardless of gender. Again, it has the same spelling and pronunciation as the feminine article, but you should regard it as a different word.
In both cases Unterlagen is the plural of Unterlage so everything is inflected according to the plural. That's why sind is used both times instead of ist. The word Unterlage itself is feminine, but in most cases inflections only distinguish gender for singular nouns, so you don't really need to know the gender in these examples.