Trouble Editing PDF with Preview on Big Sur

Hi,

Sometimes, it appears to be randomly, Preview won't allow me to highlight/underline text, move text boxes, shapes or drawings around. This gets very problematic when I take notes in class bc I can't highlight parts of the pdf and the notes get all crumbled in the middle of the page since I can't move the text boxes.

I'm using google drive on my macbook so that my files are backed up automatically. At first I thought that maybe this was the problem, so I opened a file that wasn't backed up by google chrome but still got to the same point.

When I quit the app and open it back up, it seems to allow me to use it juste fine, but it is constantly susceptible to crash and need me to quit and open my PDF again. Moreover, a pop up window saying something like "another app has modified your file, you can try to save your work so that it crushes the other version, restore the other app's version or save under a new name". Anybody can help?

BTW, I do select "text selection" under the 'tool' tab.

Thank you so much for your help,