If your PI was alive would you ask to see the letter ? You knew them, so would they be reluctant to do so or would you be putting them in an uncomfortable position ? Just because they're dead doesn't mean you should ignore their wishes as you understand them.
I ended up getting the scholarship, so I'd think the letter was quite strong,
That's an assumption, but not necessarily true. They might have been impressed by you despite what she said. So there's one reason to not read it - you might be dissappointed.
A second reason might be that maybe she said things that were good, but not true. People do that sometimes if they have a personal bias. If you find out that happened, do you then tell the scholarship people and maybe you lose what you have.
Letters like these are for a formal purpose. Maybe you want to see something that shows a personal warmth, but it was written for a formal purpose and probably isn't going to have that context. It could leave you cold and no more informed about what your PI thought about you outside of a technical and work context. The letter was never written for your eyes or even for the eyes of your PI's friends.
This is a pandora's box situation. My view would be to leave it alone.
and it would just mean so much to me to see what my former PI said. (I know I don't have any real reasons I need to see the letter besides sentimental ones, but still...)
Another expression is "curiosity killed the cat". Just how curious are you ?
My former PI (and role model and one of the most incredible people I know) wrote me a letter of recommendation for a scholarship before she passed away. I know her secretary was the one who submitted the letter (so presumably she still has it), so I'm wondering if it's ok for me to ask to see the letter.
Ask if you wish. It may or may not be allowed depending on jurisdiction and other legal factors. If you ask it may even be something they have to do, but not necessarily something that will make you look good,
You have fond memories of your PI. Keep them. Maybe they're more important to keep intact than satisfying curiosity.