\pagebreak vs \newpage

Both commands start a new page.

If the optional argument is not used, \pagebreak will start a new page and the paragraphs of the old page will be spread out so that the old page will not look like the end of a chapter.

With \newpage, on the other hand, the old page will have the blank space at the bottom, because the paragraphs will stick together as if the chapter had ended there.

This link will be useful, and the answer to a duplicate question (that was migrated from SO to this site later) gives the following example:

\pagebreak tries to make the page the same height as other pages if it's possible (by stretching intervals between paragraphs etc) and \newpage just fills the page with empty space.

\newpage forces a new page at the point at which it occurs.

\pagebreak without options forces a new page at the end of the line in which it occurs.