I don't know if this is possible (a work-around is fine), but I thought I'd throw it out there.
We would like students to practice filling in a tax form (just numbers, no text), with the form displayed in a quiz question, and they type their answer into boxes. And then Moodle grades it automatically.
We have interactive (type-into) PDFs for the forms, if that helps.
Could we have the pdf as a background image, and somehow specify where the short-answer boxes go? (so as to align them with the boxes on the form, to simulate filling it in.) How would you do that?
Any ideas gratefully received! We are using 1.9.
Cloze question type. You can convert the form to html and replace the text boxes with the Cloze shortanswer or numerical fields. See docs for syntax and examples. 
As Itamar said, doing this as a Moodle Quiz with embedded cloze questions is probably the quickest and easiest way to do this.
If you want it to look exactly like the PDF form then I'd recommend creating a software simulation with Adobe Captivate or one of the other Flash based authorware programs. They publish to SCORM, however, so it takes some very specialised, highly skilled developers to do anything other than use templates, so make sure that the software that you use can do everything you require before starting a project. Captivate 4 can definitely simulate anything a PDF can do.
It's possible to create PDF forms with Acrobat Pro but this is typically labour intensive. I haven't tried it with Acrobat Pro 9 so they might have made it easier. The problem with this approach is that you'd have to set up a XMLSocket server to collect the data and send it to Moodle which is notoriously difficult to do.
I hope this helps!
If you want it to look exactly like the PDF form then I'd recommend creating a software simulation with Adobe Captivate or one of the other Flash based authorware programs. They publish to SCORM, however, so it takes some very specialised, highly skilled developers to do anything other than use templates, so make sure that the software that you use can do everything you require before starting a project. Captivate 4 can definitely simulate anything a PDF can do.
It's possible to create PDF forms with Acrobat Pro but this is typically labour intensive. I haven't tried it with Acrobat Pro 9 so they might have made it easier. The problem with this approach is that you'd have to set up a XMLSocket server to collect the data and send it to Moodle which is notoriously difficult to do.
I hope this helps!