Most graphics formats can be exported and saved as PDF documents without any trouble. The same goes for special .acorn files from picture editor for Mac of the same name which can be exported as .pdf files using the programs default function.
If you do not have access to macOS / OS X platform and Acorn program, you will have to use some graphics viewer or editor for Windows that supports .acorn files, otherwise you really have no alternative for acorn to pdf conversion.