What file format do you recommend to save a signed transaction to broadcast much later?
Any text-format will do. There's no need to make it more complicated.
Unless you want to make a QR-code out of it, in that case I'd go for PDF. Or just save it as GIF.
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.
Like this:
Signed Recovery Transaction
010000000126063a71fecd0c76fa7d31a561cbe95bf2c7e5da01ae04fed6a6e9dfbc80953e010000006a47304402201a16d89264518baca8f4959b446372c6ce91e8d1fbc0b7b48618aeb76113df33022040eb804bf7cd6519d01709066658251cef1822ff49fd07707e058a07b27b42f9012103f78766b4346bcec0f2ae92d7e132e6b321c47627f14356a704b3ce57169dcb4e000000000116260000000000001976a914cfdd1b997472bd0b668e7472d9708305f116994d88acc0270900

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.
See
Using Locktime for inheritance planning, backups or gifts 
Or play around with
Coinb.in (don't enter any private keys on a LIVE website!).