Does Overleaf support pTeX?

Yes, Overleaf can compile Japanese documents that require pTeX. Here's how:

1. A pTeX document does not need any special commands for Japanese text (kanji, hiragana and katakana). Here's a simple example (using the jsarticle document class):

   \documentclass{jsarticle}
    
   \bibliographystyle{jplain}
   \title{p\LaTeX\ 実験}
   \author{林蓮枝}
    
   \begin{document}
    
   \maketitle
    
   \begin{abstract}
   本稿では、文書組版システムp\LaTeX{}の使い方を解説します。p\LaTeX{}を利用するときには、あらかじめ文章中に\TeX{}コマンドと呼ばれる組版用の指示を混在させ\ldots
   \end{abstract}
    
   \section{導入}
   こんにちは世界！\cite{確率統計}
    
   \bibliography{refs}
   \end{document}

2. If you are uploading a .tex file from elsewhere, make sure it is saved in UTF-8 encoding.
3. In the Overleaf editor, click on the Overleaf menu button above the file list panel.
4. Set the "Compiler" option to "LaTeX".
5. Add a blank file to your project, and save it as latexmkrc.
6. Add the following line to your latexmkrc file:

$latex = 'platex';
$bibtex = 'pbibtex';
$dvipdf = 'dvipdfmx %O -o %D %S';

This blog post (in Japanese) by doraTeX has further details about using pTeX and upLaTeX on Overleaf.