Do I need a new PR travel document every time I am abroad without a valid PR card?

Yes, in effect. A PR travel document is generally issued for a specific trip back to Canada, rather than functioning as an ongoing card that covers repeated future travel the way a valid PR card does. Without a valid PR card, each trip requiring travel to Canada generally means applying for its own PR travel document beforehand, rather than relying on a document obtained for a previous trip.

This is one reason travelling frequently without a valid PR card becomes increasingly impractical over time — each application takes its own time and documentation, and relying on repeated one-off travel documents is considerably less convenient than simply keeping a valid PR card on hand, which functions as ordinary travel documentation for as long as it remains valid.

Anyone who travels regularly, or expects to, should prioritize renewing their PR card well before it expires rather than planning around repeated PR travel document applications, since a valid card avoids this recurring process altogether and is generally the simpler long-term solution.