What you're asking for isn't that easy to do. If a visitor of your site can read a PDF in their browser, depends largely on two factors: Whether the browser can display PDFs at all, and how the browser is configured to deal with them.
Current versions of most browsers can display PDFs natively. But older browsers are still around. Some of those older browsers can't handle PDFs at all, some use a plugin, some hand PDFs over to an external reader software. According to the statistics of a website I manage, more than 40% of all visits are done with browsers that have a plugin for PDF installed.
The second factor is how your visitors have configured their browsers. With most browsers, you can configure something like "for this type of content, do this; for that type of content, do that". So if a visitor of your site configured their browser to download content of type PDF, there's not much you can do about it ;)
What you can do: You can have your site suggest what the browser and/or the person sitting in front of the screen may do. For this purpose, you might want to read up on the HTTP header Content-Disposition
. Whether your visitors follow that suggestion or not, is up to them and the software they use.