I just read your post over again and spotted that the way you’re trying to access your file is incorrect. Node-RED (thankfully) doesn’t have access to arbitrary files inside the home folder of the user who runs it. You need to put it in the static files directory, then build the correct link for node-red. You should be able to find that by a forum search.
Old reply below:
Is your goal to display the PDF within the dashboard, or redirect to the file? If to redirect, use a link (<a href="Address here">) instead. If to embed, take a look at pdfjs, a Mozilla library that is a PDF viewer written fully in HTML5
