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.
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)