LibreOffice Different fonts and text styles, embed images. Can save files in an Open Document Format (ODF), a number of Word formats, and export as a PDF.
Includes a "Calc" program for creating spreadsheets, "Impress" for slideshows, "Math" for mathematical equations.
Notepad++ You won't be able to format what you write. Useful when editing code and basic text. Auto-complete feature.
Other open-source options
LibreOffice is descended from Apache OpenOffice, which is similar, but updated less frequently.
For typesetting, you might consider LaTeX.
If you're looking to take notes, you may want to try Notable. Newer versions of Notable are no longer open-source.
Free? But how?
These programs are free, so how do people who work on them make a living? Sometimes, developers tinker with open-source software part-time as a passion project, but in other cases, people actually find a way to make a living working full-time on FOSS (free open source).