Can’t say anything. Since, t use SSL, so no any third party can happen to be in between and steal your valuable document. But the question is whether services like pdftables are safe or not. T say t are. But in reality no one knows what is happening behind the servers, whether t are saving and reselling or t are being honest and deleting. The safer alternative is to use apps like Scanne, t don’t process your pdfs on their servers t process them right on your phone.
And have a great day.