There are certain file formats that are explicitly designed to be archival safe, like PDF/A. For long term stability, I always try to export a copy to one of those.
File formats like .doc or .psd that are tied to evolving product lines are bound to change eventually. They're great for working copies, but I don't expect those file formats to still be readable in 10-20 years. Unless the file format explicitly calls out long-term archival as a feature, I assume a PM will eventually change the file format and break things.
You can still read doc files from 20 years ago. Microsoft is very good at keeping backwards compatibility. I would be surprised if they decide to change course and start breaking things.
It's a tough problem. The original Microsoft Word file format wasn't even really a file format in the way that modern developers think. It was basically just a dump of raw memory blocks to disk, with the necessary adjustments to pointers between blocks. That made it very fast to save and load files on old hardware. But using such formats with modern code bases is extremely difficult and error prone, even if you have all the original source code.
File formats like .doc or .psd that are tied to evolving product lines are bound to change eventually. They're great for working copies, but I don't expect those file formats to still be readable in 10-20 years. Unless the file format explicitly calls out long-term archival as a feature, I assume a PM will eventually change the file format and break things.
[1]: https://en.wikipedia.org/wiki/PDF/A