Problem of using Preview

Im having some problem with Preview lately where I cant see my editing on the PDF slides when i open it next day on my mac or iPad, its like parts of my notes are gone like it is half way done. Im sure that i have saved the file every time, this has become a big trouble for me especially I'm doing online classes now and I'm relying on Preview to jot down my notes. At first, i thought it was software issue or iCould problem, then i upgraded my old mac to latest version (Big Sur) but the problem still exist. Could anyone help me out on this or anyone is facing similar problem?