Alright,I understand now.Thank you !

They are not interchangeable. We use 펴다, 접다 with a book. We don't use 열다 or 닫다 with a book. The difference is due to the register(언어사용역). 열다 is used when we can see space after the action. After 문을 열다, we can see the space of the room After 가방을 열다, we can see the space of the bag. After 책을 열다, we cannot see any space in the book.

But I've seen 펴다 used in the expression "책을 펴세요" which I think means open your book.That's why I was asking 펴다 = to open,열다=to open and what is the difference.Is there any situation where you could use them interchangably?

펴다(unfold) is antonym of 접다.(fold) 열다(open) is antonym of 닫다.(close)