Problem with viewing iCloud files on iPhone

Hi

I am using iCloud to synchronise files on MAC so that I can view them on my iPhone (Files)

The thumbnail of the pdf format file is always shown but when I try to view the file it does not display properly.

This does not happen with all pdf in the same folder. This problem also applies to other folders

Once I select the file, instead of the pdf file displaying, I have a grey screen with the name of the file and below it the word "data"

My iPhone is a 6S Plus iOS 11.2.6