Pdf documents can be displayed in SharePoint site with the use of “Content Editor Web part”. To know more about "Content Editor Web part", I suggest you to read this post, where you will get to know about playing a video in SharePoint site.
Add a “Content Editor Web part” to your SharePoint page, Click on the “Open the Tool Pane”, click on “Source Editor” to place the HTML source.
Place the following piece of code in to the Source Editor
In the above code, “src” points to a source location, the location can be a HTTP or a UNC path. I’ve stored my Pdf document in my document library and used the Http location of my Pdf file. Also Set the Height and Width properties and click on “Ok”.
The above image shows the embedded Pdf document to a SharePoint page.