Once I had created blank files in the data root the final set of errors it created was:
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/sad.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/sad.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/sad.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/sad.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/sad.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
Warning: imagecreatefrompng(): '/home/www/moodledata/temp/assignfeedback_editpdf/stamps/cf35df2a4476b32c42c3a3d9aed9ad1523011e3c/cross.png' is not a valid PNG file in /home/www/scvlp.co.uk/lib/tcpdf/tcpdf.php on line 7007
And it now creates annotated PDF for student