This appears to be trying to solve a problem that doesn't exist with a solution that is more problematic than the problem it attempts to solve.
If my browser can handle pdfs, I probably want to view them using the browser[1].
If my browser can't handle pdfs, it will be forced to download them anyway.
If my browser can handle pdfs, but I want to save it/view it with another program, I can use right-click+save as.
By contrast, a pdf which I want to view in-browser, but which forces the browser to download it instead is just a nuisance. Even if there were a 'right-click+don't save it, just view it in browser', I don't know that I need to use it until I have tried clicking the link.
In general terms: You publish content on the web, I decide how I view it. Try to force my hand and you'll just annoy me into going elsewhere.
[1] If the browser supports pdfs by default, but I prefer to view them with somthing else, there are usually settings to tell the browser how to handle different file types.
If my browser can handle pdfs, I probably want to view them using the browser[1].
If my browser can't handle pdfs, it will be forced to download them anyway.
If my browser can handle pdfs, but I want to save it/view it with another program, I can use right-click+save as.
By contrast, a pdf which I want to view in-browser, but which forces the browser to download it instead is just a nuisance. Even if there were a 'right-click+don't save it, just view it in browser', I don't know that I need to use it until I have tried clicking the link.
In general terms: You publish content on the web, I decide how I view it. Try to force my hand and you'll just annoy me into going elsewhere.
[1] If the browser supports pdfs by default, but I prefer to view them with somthing else, there are usually settings to tell the browser how to handle different file types.