Add the PDF to the SHFB project with a Build Action of Content. That will embed it in the help file. You can then use an anchor link to open it. For example:
<remarks>See <a href="../Test.pdf">Test.pdf</a> for more information</remarks>
Note the relative path which is required since the HTML pages are in a subfolder. If you put the PDF file in a subfolder in the project, just add the subfolder to the relative path as well (i.e. ../Subfolder/Test.pdf).
It is possible to access an external file from a Help 1 file using the "file://" protocol and a fully qualified path like this:
<remarks>See <a href="file://C:\DotNet\Test\Documentation1\Test.pdf">External Test.pdf</a> for more information</remarks>
However, the fully qualified path must be the path on the system on which the help file is viewed, not just the one on which it is built so that can be a problem unless you know that the external PDF will reside in a common location on all possible systems
on which the help file can be viewed.
The first method will work in Help 1 and website output. You can add a target="_blank" attribute to force it to open in a new browser window in web output. The second method will not work in website output due to the local file system
path. Neither method will work for MS Help Viewer output due to the way it reformats the URLs.