I synched my Google Calendar with the iOS Calendar app (iOS 10.3.2) by adding my Google account. I can see the attachments and file size in the event, but when I click on it to view the PDF file, it brings up a gray screen. Multiple attachments to multiple events do not work. I can share or click Done. The attachments work in the desktop web site.
I can see the attachments in the Calendar event on my phone, but when I click on them, it only brings up a blank page without any content. The file size shows in the list of attachments too.