PDF damaged or corrupted

I have been writing notes on a pdf-book using pencil on my iPad. Used to open pdfs in default pdf viewer. Today when I was scrolling the pdf the file closed automatically and now its not opening. The size also got reduced to 17 mb from more than 1.5 gb (it was a 950 page book).

Also in the iCloud it got synced and I'm seeing the same corrupted / damaged file!

Is there a way to recover the file, I have my exams really soon and all my notes are gone. Please help

Apple iPad Air (5th gen)