What file format do you recommend to save a signed transaction to broadcast much later?

I suppose I should save it in a text file (.txt) or in a pdf file using just plain text with JSON or XML.

Actually, I don't have a clear idea of how does a signed transaction ready to be broadcasted and validated by the bitcoin network (and eventually entering the blockchain) does look like in plain text.

Even less, how does it look like if it uses smart contracts with "Bitcoin Script".

But I want to learn about it. For instance, it would be very useful to be able to lock some coins and would be sent to someone else's address in the distant future if I did not change my mind in the meantime. This way I would know that if I die someone from my family would receive my coins in the distant future as I would not be able to change my mind after dying. Hope it makes sense.