From now on, when you open PDF files in the Google Drive app for Android, new editing tools appear. In particular, a pen for drawing and a marker for selecting text. You can change the color of the tools (red, black, blue and green for the pen and yellow, green, blue and purple for the marker) and the thickness of the lines (from 8 to 40 pixels).
Edited PDFs can be saved as original files or as copies. New functionality appeared in version Google Drive 2.23.081.2 for Android.