Because PDF files are nearly universal, unless the computer was put together in the user kitchen it will have a PDF reader installed as it comes from the manufacturer
Many instructional files and computer help files are in thr PDF format.In some Genealogical pages much of the content is in PDF format. With today's computers and internet it is quite limiting not to have PDF readers.Adobe is the main provider of PDF readers, but there are many others.The Adobe PDF reader is free, as are some others.
Unless they are not connected to the internet, and they have gone to the PDF reader and turned off automatic updates, they probably have the latest version on their computer.
The RTF format assumes the user has a MS Office or MS Word installed on their computer. While most major word processor can import RTF files there are some inexpensive ones that can not. In today's computer world it is an obsolete format.