The easiest way would be to use an HTML Iframe and change the source of the iframe using the target attribute in the link's <a> tag. This will work, with one big caveat, no matter what you do with iframes, modal windows or javascripts to get multiple PDF files to show the way you describe, the user's browser settings can easily override your embedding methods.
As developers, we really don't get to decide exactly how media files like PDFs are displayed. If the viewer of your site has "Open PDFs in Acrobat" as their default setting, it will download and open in their Acrobat program leaving the webpage with a broken appearance (blank white space next to links).
Because of the limitations like this, many sites will simply link to the PDF file as if it were another page of the website. For those users who have their browser set to open the file, the browser will open it. For those who don't the file will download and open in Acrobat, rather than showing a "broken" web page.