www.wysiwygwebbuilder.com

I am experimenting with adding .PDF files to display pages - https://rvmha.info/?page_id=1755 - and the background is defaulting to Black. I would like it to be white.

Is this something that can be specified in WYSIWYG Webbuilder or a function of the pf reader install on my PC?

First please note there is no support with sites not made with the software. You haven't said what browser you are using?. If using chrome then this explains why due to it will render black if the PDF page itself has no background color.
Another possibility is that the PDF viewer/reader is using “dark mode” or "transparent‑background" this will then inturn render anything in the PDF that doesn’t explicitly have a white background will appears black.
A solution in the software is to use an iframe to display the pdf and set thr iframe to white this will then display any transparent areas of the pdf white as the background shows through.