There is no problem in using a PDF filler.
I have read on this forum peope advising against using a PDF filler and yes the form does state to use black ink in block capitals, but there is nothing on the form that prohibits having the form filled in electronally.
A number of people including myself have used electronally filled in forms for PEO appointments and this has not been an issue. I had my other half's ILR at the end of May 2015 and it was not even questioned so using an electronically filled form is not a problem.
My experience and a couple other examples can be found on page 14:
http://www.immigrationboards.com/indefi ... 4-260.html