I have designed a theme portfolio. Now, should I save each screen on different page in a single pdf, or all on a single page of a pdf file?

The answer is yes. Do what you feel is the best presentation.

Samsung Developer Program

Rejected my theme. Sir tell me what is missing in my theme and what improvements can I make and apply again

Make it more unique. It’s to simple and not unique enough. You must create a Theme, that the Review Team see it and feels. Wow we never so something like this