Thanks for the response Thomas,
I am using a different group for the PDF gallery content on my "PDF Maps" page, not the "Content" group that was automatically created for my site. Same goes for my "Interactive Maps" page I created, the content in that associated gallery also is not the automatically created "Content" group for the site (but this has posed no issues). I wanted to narrow down my site search to only include downloadable open data like shp/kml/xls formats and keep the search separate from my PDF and interactive maps. As a result, PDFs aren't even being displayed as options to choose from in my search.
I tested again myself and every PDF was working perfectly fine in view mode (AGOL login credentials I assume are still cached as a result). So I shared the "PDF Maps" page publicly, tested in an incognito window, and noticed the PDFs no longer would display for me at all in incognito. I got a coworker to try with the site momentarily shared publicly, and found both on Chrome and Edge (with no cached AGOL credentials) that the PDFs would not display for him either. Then magically after trying a few times, the PDFs would show for him on both Edge and Chrome. He was not signed in, and in incognito when I tried neither was I. We never use Edge organizationally either so there should not be anything related to AGOL in his Edge cache.
Interestingly enough, I tried sharing one of the PDFs with the site's default content group, and that PDF now shows for me in incognito. Then I removed it from the default content group, and it would no longer display in incognito. Toggled back and forth a couple times and got the same results each try. Do I just need to accept that I will have to share all PDFs with the default content group and that they will show up in my search? It's weird to me that my coworker can sometimes open all PDFs from the PDF page (while not signed in), even though the PDFs aren't shared to the site specific content group. I could not see any when viewing publicly in incognito comparatively.
A lot to unpack here, and I'm probably over complicating things. Thanks for your help!
Brett