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 "Settings" button (i.e. the gear icon next to your username in your project window).
4. Under "Advanced Build Options", from the dropdown list, select "LaTeX dvipdf". Then click "Save settings".
5. Add a blank file to your project, and save it as latexmkrc.
6. Add the following line to your latexmkrc:

$latex = 'platex';
$bibtex = 'pbibtex';