You can now work directly with PDFs received in the Mail app using a little-known feature in iOS 18 and iPadOS 18. Tap a PDF attachment in a message and use the Markup and Form Fill buttons at the bottom to access the PDF markup and filling tools. After modifying your PDF, tapping the Done button gives you options for what to do with the PDF: include it in a reply, create a new message with it, save it to Files, or discard the changes.
(Featured image by iStock.com/chanakon laorob)