Hi @twhiteside!
Welcome to the community! 
So, sadly, I was not able to modify the preload file in a way that persisted the changes that allowed for the pdf to render. Rather, it was only a one time thing I could do. I can change that .html postman generates, open it in Chrome, and then it would show the pdf. However, When executing the request again, a new .html file is generated, and so any changes I made to try and persist the information did not work.
You can find the preload files by referencing the article here, for debugging visualizer.
You will notice when “Inspecting Visualization” (right click in your visualization when it loads), it just opens Chrome developer tools. Then at the top, of the new window for Developer Tools, you’ll see its referencing a file path to an html. file. Now sure what it is in Windows, but for macOS I found it under /var/folders/ and then some random string folders afterwards.
Update:
I did make a video on visualizing PDFs with Postman. Sadly, its not within Postman, but you can set up the HTML in Postman and then view the PDF in a browser:
In addition, I started an RFC for natively supporting browsers in Postman Visualizer:
https://community.postman.com/t/native-support-for-pdf-files-in-visualizer/17631
Now it just needs some traction 
Hope this helps!