If you use the Gmail app, some of the PDFs don't open. When the page you get looks like this
tap the "Share" button at the upper right. This will show all the apps that can open the file. Select/tap the one you prefer, look at the PDF file there.
It's also possible to save the file to Google Drive without opening it. Tap the Google Drive icon on the still closed file.
Open the app afterwards to view the PDF file there.
The email actually contains a mime-app. When you tap it, the pdf file appears. When I tap the pdf file, it will not open.
Sorry for being confusing.
Thank you for any help you can provide me.