You have been over two days waiting for a response so I will have a go as nobody else seems willing to try. I have never used AES encryption or a database Blob but I will do my best.
1. It is possible to save large size file, When I save 20MB size file Out Of Memory error occur
I wouldn't call 20MB a particularly large file, and since your device has many gigabytes of memory it is difficult to imagine that it has run out of space. It is far more likely that there is an error in your coding but it is hard to be more specific when you don't show us either your code or your error message.
2. How to convert Bytes to File without using Filename and Dir.
When you save data in a Blob you are saving an array of bytes. When you retrieve the Blob you get back an array of bytes. You have got a problem with your "mental model" I think. If the data represented an image then you could load it into an ImageView. But it sounds as though this data represents a PDF, so I would expect that you would want to convert it back to a PDF. If not, what do you want to do with it?
A final suggestion - don't ask two different questions in a single post. It would be better to start again with two separate threads. And show some code.