Plain text, with AsciiDoc
Dr. Matt Neuberg writes very long, very technical books using plain text editor. He marks up his copy with AsciiDoc tags. Converter tools can then transform into PDF, HTML, and various ebook formats. He explained his tool chain.
This post mentions some other authors who do the same.
Plain text, with Markdown
Ian Hocking does similarly for writing novels, but using markdown tags, as he explains.
As commented by Izzy, the LibreOffice suite and its module for word-processor should be able to handle a 50-page academic paper.
LibreOffice would be my first choice. Explore the plain text tool chain above only if you expect to do much more writing of even greater length.
Some huge tips:
- Learn to use style sheets. Never apply styling/formatting manually.
- Backup often. Any word-processor will eventually destroy your document. I offer my grey hair as proof.
- Install no upgrades in the middle of your project.