PDF’s which require a password to be opened cannot be attached to any object.
What is the extension of the file you are trying to upload?
It is a .PDF
I just created one and could upload, i’m leaving here the file.
https://drive.google.com/file/d/1mlXTUT70eO0XvjiR40paKT9aoyDILQ_I/view?usp=sharing
i used something like this to create it:
kisero-2:Desktop kisero$ zip --password llllll sample.zip sample.txt
can you try the same? what software are you using to compress with a password?
sorry…i had zip in my head and you were talking about a pdf 
i encrypted a pdf on my mac and was also able to upload it
the file: test.pdf - Google Drive
same questions as wiht the zip example apply tough !