The LaTeX beamer class is optimized for presentations using a video projector (thus the denglish term beamer). The aspect ratio is 4:3, and the page size is small to exclude problems with ancient projectors. As a matter of fact, beamer class presentations should work with any pdf viewer and any video projector no matter the age of these soft- and hardware components.

Can we, however, use the presentation also as a handout, or as a poster? And how?

Easy: 😉

Here's the poster,

%works together with option trans in documentclass
\pgfpagesuselayout{resize to}[a4paper,landscape, border shrink=5mm]

and here the handout:

%works together with option handout in documentclass
\pgfpagesuselayout{4 on 1}[a4paper, landscape, border shrink=5mm]

An example for a handout: