The PythonAnywhere help pages offer information on working with PDF files here: https://help.pythonanywhere.com/pages/PDF
Summary: PythonAnywhere has a number of Python packages for PDF manipulation installed, and one of them may do what you want. However, shelling out to abiword seems easiest to me. The shell command abiword --to=pdf filetoconvert.docx will convert the docx file to a PDF and produce a file named filetoconvert.pdf in the same directory as the docx. Note that this command will output an error message to the standard error stream complaining about XDG_RUNTIME_DIR (or at least it did for me), but it still works, and the error message can be ignored.