Peter Wells discusses one of Spark’s features:
But the killer feature for me is the way Spark tackles signing PDFs. Spark will open email to mark up in Readdle’s PDF Expert (which is free for basic editing), and when you’ve scribbled, the PDF is attached to a reply email to the sender. This is such basic, obvious behaviour, but no other email client on iOS behaves that way. Most force you to save the PDF to the Files app, then jump back to the email and reply there, or send a new, completely out of context email.
I wrote about Spark recently and didn’t discuss the feature, as I just found it myself while fiddling with the app during a meeting (ahem). Spark is free unless you use the collaboration features, but I’d happily pay for the app.