Most PDFs are not possible to open in Browser with iOS.
It is necessary to open file with other Apps.
The icon for the PDF-File is different as the normal PDF-Icon.
Maybe you have some example PDF, because in my tests it’s working fine with PDF files.
Sure!
It is not possible to add PDF files on this post 
Hi @goldstony. Could I ask you to create an issue report on GitHub with a sample file, please? There it should be possible to attach a sample PDF file. Thanks a lot!