This is true. If you open a PDF in iOS, there is a little toolbox in the upper right hand corner which, if you tap it, reveals a number of editing tools. Those tools include pasting in a signature that has been pre-written, or signing the document with your finger or the Apple Pencil. I've use this markup feature in the past and it is especially useful for email attachments that require my signature. All you do is open the attachment, sign it, and then you can respond to the email and the signed document is attached. The whole process takes about 10 seconds.