How do I adjust (over an entire document) the length between paragraphs to make it smaller? (The same way linespread adjusts the space between lines)

It depends on what it is set to from the beginning. There are two lengths defining how a new paragraph is marked, \parindent and \parskip. The first one sets the indentation of the first line and the second the extra distance between paragraphs. A new paragraph should normally be marked by either indentation or an empty row, but not both. By default (at least in article, book and report) they are set to

\parskip=0pt plus 1pt
\parindent=15pt


which means that a paragraph is marked with indentation. The setting 0pt plus 1pt on \parskip means it is a rubber space (skip), that it is 0pt but can stretch to 1pt, i.e. very small.

Aparently, since you are asking to decrease the skip you have some other settings. You can find out the settings by including in your code:

\verb|\parskip|:\the\parskip\ and \verb|\parindent|:\the\parindent


Both can be set with \setlength to whatever you like. So to change to have an empty line and no indentation you can write e.g.

\setlength\parskip{1em minus 0.2em plus 0.1em}
\setlength\parindent{0pt}


It is often recommended to have a rubber space in vertical distances like \parskip, hence the minus 0.2em plus 0.1em that tells teX how much it can stretch (negative and positive). These settings should be done in the preamble (before \begin{document}).

\setlength{\parskip}{12pt} will help you to fix...

\usepackage{parskip}


this solves the problem, inserts a space instead of indentation