Free and Open Source Software(FOSS) a software which is both free and open source.

“'Free software' means software that respects users' freedom and community. Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software. Thus, “free software” is a matter of liberty, not price. To understand the concept, you should think of 'free' as in 'free speech,' not as in 'free beer'" - Free Software Foundation1.

FOSS programs are frequently audited by security researchers and are very secure. Using FOSS alternatives can improve your security as you give a program/software necessary permissions to access some of your data for providing you the service. Proprietary software does not share information about how it works and as such cannot be trusted.

If you are using Linux, then most of the programs you use are free and open source software. But when you are installing new software check their licenses to know whether they are FOSS or not. Read more about licenses used by free software here 2. But if you are using Windows or Mac, choose for the FOSS alternatives from the below list and from the extra resources listed.

1.https://www.gnu.org/philosophy/free-sw.html
2.https://www.gnu.org/licenses/license-list.html

List of FOSS alternatives

Media Player

VLC Media Player (Available for Linux, Windows, Mac, Android and iOS)

Office suite

Libre Office (Available for Linux, Windows and Mac devices)

PDF reader

SumatraPDF (Available for Windows devices)

Web Browser

Firefox (Available for Linux, Windows, Mac, Android and iOS)

Resources to find more FOSS alternatives

PRISM-BREAK - Privacy-aware recommendations to proprietary software
privacytools.io - privacytools.io provides knowledge and tools to protect your privacy against global mass surveillance
AlternativeTo.net - Great collection of open source online and self-hosted software sorted by likes