\flushbottom ¶Make all pages in the document after this declaration have the same height, by stretching the vertical space where necessary to fill out the page. This is most often used when making two-sided documents since the differences in facing pages can be glaring.
If TeX cannot satisfactorily stretch the vertical space in a page
then you get a message like ‘Underfull \vbox (badness 10000) has
occurred while \output is active’. If you get that, one option is to
change to \raggedbottom (see \raggedbottom). Alternatively,
you can adjust the textheight to make compatible pages, or you
can add some vertical stretch glue between lines or between paragraphs,
as in \setlength{\parskip}{0ex plus0.1ex}. Your last option
is to, in a final editing stage, adjust the height of individual pages
(see \enlargethispage).
The \flushbottom state is the default only if you select the
twocolumn document class option (see Document class options),
and for indexes made using makeidx.