How to scale all fonts (in the whole document) by certain factor or percentage?

You could load the relsize package and issue the command

\relscale{0.95} % or whatever scaling is desired

at the start of the document.

it depends to the font if it is scalable or not and if there is a LaTeX support. It is easy if you can use OpenType fonts with xelatex or lualatex. With pdflatex you can scale down the complete output. But this influences also all lengths, like \textwidth:

\documentclass{article}
\usepackage{lipsum}
\AtBeginDocument{\pdfliteral { 0.95 0 0 0.95 0 0 cm }}% set TMatrix
\begin{document}
\lipsum
\end{document}

Why not add \linespread{0.95}, it reduces the space between two lines with 5%, so there will be more text on your page.