Unfortunately HMRC does not provide a 64-8 that you can type on (warning: PDF file), which means that if you want to email a client with a 64-8 to sign, you have to print, fill in by hand and scan and email.
This is unsatisfactory from a professional and aesthetic point of view.
If you have the paid upgrade of Adobe Reader or some other PDF software then you can use the typewriter tool to type on the form. However its a lot of money for one purpose.
Fortunately I have created a PDF of the form with fields to fill in by using PDF Escape.
Here is the Typeable 64-8 for your use (at least until HMRC changes the layout again).
(Remember that it is probably still easier to use the form rather than the on-line authentication if you are being authorised for more than one category: CT, VAT, PAYE etc.)
You may try pdffiller.com Let’s u type on top of any pdf form
I think PDFfiller creates an image of the PDF that you type on and then another image of the finished work. For a long form this can make either the document very large in size or of a poor quality.
The PDF software I mention preserves the PDF so that text can scale and be extracted or read by a screen reader rather than bitmapping everything.