Example using xr package on Overleaf
This project demonstrates how the xr package can be used on Overleaf to resolve labels in other .tex files that have not been \include-d in the main file.
Make sure you enable the correct system call in latexmkrc, depending on the LaTeX engine you're using!
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
Using PSTricks with pdflatex on Overleaf
PSTricks drawings can't be compiled under pdflatex and lualatex (Frequently done errors #5). Instead, compilation with XeLaTeX is preferred if you need to use .jpg/.png/.pdf images in your project.
But if you absolutely need to compile with pdflatex, this example demonstrates how to use PStricks with pdflatex using auto-pst-pdf on Overleaf (But you still can't use .jpg/.png/.pdf images within your PSTricks drawings).
An example on using pTeX on Overleaf. See this help page and blog post for further details on using pTeX on Overleaf.