OK here's a new one I would really appreciate the community's ideas on this.
When I play the app with a PDF viewer of a SP library it functions as desired! I followed the video by Rezi Doranni (
PowerApps PDF Viewer SharePoint Document Library without flow) and it worked perfectly.
For my testers:(with READ permission in SP) all files types render perfectly, except pdf
They see this:

Why would this only be on PDFs?
I tried changing their permission level in SP to CONTRIBUTE, but it didn't help.
As for the "HTTPS" prompt, I added it and it works great for me when I use the app link for users, but not for anyone else.
Here is the code for the viewer.
If(
Last(
Split(
GalleryCostLibrary1.Selected.'File name with extension',
"."
)
).Result = "pdf",
"appres://datasources/OUR SP/table/c0d8c0b5-3495-45be-ad6d-a3e112dea6a7/rows/4/reference/https:%2F%2Fmsmanaged-na.azure-apim.net%2Fapim%2Fsharepointonline%OURENVIRONMENT185a6bf6%2Fdatasets%2Fhttps%25253a%25252f%25252fOURENVIRONMENT.com%25252fsites%25252fusa%25252fSiteName%25252fSiteName%2FGetFileContentByPath%3Fpath=%252f" &EncodeUrl( "https://OURENVIRONMENT.sharepoint.com/sites/usa/SiteName/SiteName/"&GalleryCostLibrary1.Selected.'Folder path' & GalleryCostLibrary1.Selected.'File name with extension'),
Substitute(
GalleryCostLibrary1.Selected.Thumbnail.Large,
"/thumbnail",
"/pdf"
)
)
And the code for the PDF Viewer OnVisible that works fine:
With({fileextension:Last(Split(GalleryCostLibrary1.Selected.'File name with extension',".")).Result},If(fileextension="xlsx"||fileextension="docx"||fileextension="ppt"||fileextension="xls"||fileextension="pdf",true,false))
What would cause the difference between my clicking the app link as app owner & testers clicking the app link?
If changing the Sharepoint library app permissions didnt help.
Thank you to whomever might have some ideas.