Why is \pagebreak not working as intended?

According to this post, \pagebreak will end the current page and the paragraphs on the cut page will spread out over the page. However, when I tried this functionality, it does not seem to be working, am I missing something? The following is an MWE.

\documentclass{article}
\begin{document}
paragraph 1

paragraph 2

paragraph 3
\pagebreak

paragraph 4
\end{document}

• Try adding \flushbottom in the preamble (or switch to \documentclass[twoside]{article}). – GuM May 3 '17 at 23:04

The article class sets \raggedbottom (unless it is loaded with either the twoside or the twocolumn option). If you add \flushbottom before \begin{document} you get the spreading (and an “Underfull \vbox” warning).