Do you sometimes receive PDF documents via email or iMessage that you don't know how to fill out or sign? iOS has the tools you need. Note that these instructions are for Apple Mail in iOS. It may work in other apps as well.
Open the PDF.
Tap the Markup icon (looks like a toolbox) somewhere in the preview window. This opens the PDF in Markup mode.
Use the tools to the left at the bottom of the preview window to fill out and sign the PDF form.
Tap Done when you are done.
The tools available are drawing tool, loupe tool, text tool and signature tool. You can change appearance of all tools using the settings at the bottom to the right.
If you fill out a PDF you received by mail and then tap Done, the edited PDF will automatically be attached to a response email.
The Markup tool is available in iMessage, the native iOS Mail app and in the iCloud app and works with PDF files, photos and other images.