Hi Marie-Neige Beaudoin-Guerin,
Sorry for the late response. I have test from my side with the following steps, you may have a look and see if they help you:
Note: this way only works when you are using SharePoint modern experience.
The Download button still exists when you using SharePoint classic experience.
1.In the SharePoint site, create a new permission level which doesn't have the Download button from the library.
Go to Site permissions>Advanced permission settings>in the site permissions page, click
Permission Levels>Add a Permission Level, create a new permission level having the following permissions:


2.After create the new permission level, go to the library which contains your pm3 files.
1)Go to Library Settings>Permission for this document library>Stop Inheriting Permissions>select all existing SharePoint groups and click
Remove User Permission to remove them from this library permission.
2)Click Grant Permission, in Invite people, enter
Everyone except external users and other external users(if you need to share the library with external users).
Click SHOW OPTIONS on the bottom, select your new permission level and click Share.
Then check if your users can see Download button when they access the SharePoint library.
Best Regards,
Tina