If there's one thing that's pretty much universally hated, it's when content that you didn't ask for is shown on your device. That's what Microsoft is doing though, as users are seeing ads to install apps show up when opening or sharing a file.
This behavior happens after installing a Microsoft Office app, and according to the report from Android Police, it happens at different times. For example, if you install a Microsoft app and then attempt to open a PPTX file, it suggests installing PowerPoint as shown in the image above. Obviously, if you don't want PowerPoint, you can choose to stick to Google Slides.
Installing Your Phone Companion will show an ad to install OneDrive when sharing images. That shows up right in the share dialog. Obviously, most of this seems pretty minor. It's reasonable that if you install Word, Microsoft might assume that you'd also want Excel or PowerPoint. 'Advertising' OneDrive for those with Your Phone Companion takes it a bit further, of course. And while it seems small, it's worth considering what would happen if every developer that makes an app on your phone used these advertising tactics.