Choosing appropriate compiler

TL;DR

If you're suggested to switch compiler, you can do it with Compile dropdown.

See also

• Troubleshooting Cyrillic symbols

pdfLaTeX vs XeLaTeX

pdfLaTeX and XeTeX/XeLaTeX are two compiler engines. The former is older and is not Unicode-aware by default, while the latter uses Unicode. Documents prepared for pdfLaTeX may well be encoded in UTF-8, and in this case they are likely to have \usepackage[utf8]{inputenc} command in the preamble. If you compile such document using XeLaTeX (which is the default compiler in Papeeria) then you will see error message saying You seem to be compiling pdfLaTeX based document with XeLaTeX compiler.

Switching compiler

You can switch the compiler using options in TeX options group in Compile dropdown. Choose pdfLaTeX if your project is designed for use with pdfLaTeX.