I have this problem since april 2018, business wise. We use Salesforce and in Salesforce the List Email functionality.
When we send an Email with an attachment (PDF or Word or else), Salesforce sends the attachments as an HMTL link : https://help.salesforce.com/articleView?id=000326717&type=1&mode=1
- With the list email feature, it is possible to send files as attachments; but the email attachments will always be sent as .html files.
- To view the attachment the email recipient simply has to click on this link and the file will open in a new browser window on their computer. Recipients then have the choice to save the file locally.
When receivers open the emails with attachments on Imac, Windows Desktop, Android Phone, Samsung tablet / phone, there is no problem to open it en read it. Receivers with an Iphone and Ipad cannot open the html links, as described in this discussion.
Even if you have Google Mail or Outlook or Chrome installed on your Iphone or Ipad, you can't open the html link. The only difference is dat an url is displayed. Maybe this url gives some information? See for the url the attachment. 