How to view doc/pdf files in the application?

Question

iOS

The server can have any doc, docx, pdf file. It must be opened upon request. The method below is quite working, but sometimes it refuses to show the file, I don’t know why. Who knows an alternative or why sometimes the method works through time?
window.open(' https://docs.google.com/viewer?url= ' + url + '&embedded=true', '_blank', 'location=no');