1. Open the mail app on your iPhone (this example shows the Gmail app, but iOS Mail, etc. also work).
2. Go to the email that has the attachment. Click on the attachment to download. Once it's done downloading, click on it again.
3. Press the "Share" icon in the upper right corner.
4. Select "Save to Markup."
5. You will have the option to "Keep in original" or "Save to PDF."
6. It will begin the conversion to Markup. After a few moments, the attachment will be available in the "Extension" folder in Markup.