As other mentioned, plain text (.txt) are best format you could use. For extra caution, make sure it uses standard encoding (AFAIK it's UTF-8 and ANSI), since few application have trouble when your file has non-standard encoding.
Image file which contain QR code of signed transaction (which shown by @LoyceV) is more convenient if you're mobile user and have bitcoin wallet software which can scan QR code of signed transaction and broadcast it.
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).
Most people usually just use word (with gdrive, ms office or open office) or markdown file and convert it to PDF, it's easier than learning TeX-based format.