OnSong can transpose text-based chord charts when chords can be detected. If you are using PDF charts, OnSong cannot automatically transpose those documents because they are designed to be printed and do not contain any meaningful musical content that we could possibly transpose. If the file is a PDF and has a CCLI Song # assigned to it, OnSong can transpose those files by requesting transposed copies of the PDF chord chart from the SongSelect web service. Of course in order to do this, you need to be authenticated with your SongSelect account. Once you sign in, OnSong can request the transposed chord chart from the SongSelect service, download it, and save it to your OnSong library.
The initial process requires access to the Internet to complete. Once you've downloaded this version, OnSong can transpose to that version of the song without requiring access to the Internet.
If you are using SongSelect content, you can also opt to use their website on your iOS/iPadOS device to download and import the ChordPro version of the chart into OnSong.