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.
PDF is a great format for presenting documents for consumption... but it's notoriously difficult to copy text data from a PDF file. Blank spaces and newlines are inserted at the wrong places in the clipboard, and some characters are changed into similar Unicode characters from PDF to clipboard, and creating a PDF is a very complicated process that involves using things like TeX and *latex (and there are several kinds of latex programs such that it's confusing to beginners).
There's literally no reason not to use a plain text file for the signed transaction because it takes the smallest amount of space, is easy to copy from and paste to some transaction broadcaster, and there are no broadcasters that accept a QR code representation of a transaction as input.