It looks like you are using Samsung Notes too :) All of the sheets were originally hand drawn on my tablet in Samsung Notes, so I wonder if it has converted it back to SDOC from PDF? Very interesting!
If you want to make sure you don't accidentally erase the background, you can import as a PDF page template. This is how I play on my tablet. I guess this also means you can make mods to the sheets as you see fit, like I do :)