Hi liannawright ,
My name is Lucia. I am an Independent Advisor. Thanks for contacting us.
TWinUI — or Tablet Windows User Interface — is included in Windows 8 and newer. It’s not a program on its own, but rather a system library that is part of the Windows Shell that provides a set of functionality — including starting applications to Windows itself and other programs. TWINUI is often mistaken for a built‐in PDF viewer or email client in forums and discussion online. This is most likely because most users will only encounter the name “TWINUI” when they either open a PDF file from their browsers or click on a mailto:, call:, or other protocol link recognized by the Windows app launcher.
https://www.ctrl.blog/entry/what-is-twinui
Because you already have a program to open your PDFs, then the system knows that you should no longer use it.
I wish I had helped you.
Best regards
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