Hope you are doing well, As described, you want to use a freehand drawing tool in PDF
You may consider using the drawing tool under comment and see if that works for you. Go to Tool > Comment > in the comment tool bar click on Drawing tools. For more information please look at the help page https://helpx.adobe.com/in/acrobat/using/commenting-pdfs.html
Let us know if you experience any trouble or need more help.
We are happy to hear that, feel free to contact us for any assistance required in the future.