I would like to know if "hope you´re doing well" is used as to say hope all is well with you or when we are concerned or want to know about someone's health?
In my experience, sentences like 'I hope you are doing well' are not generally used in business letters or emails..
These questions reasonable (if this is not a business letter)
How are you?
How are you feeling today?
How do you feel today?
You could use the term "hope you're doing well" when you know that the addressee has been battling a health problem.