Best source I am aware of is the EFAST2 FAQ 33a: (https://www.dol.gov/sites/dolgov/files/EBSA/about-ebsa/our-activities/resource-center/faqs/efast2-form-5500-processing.pdf)
I don't know if you are focusing solely on signatures required of the plan administrator but, the enrolled actuary's signature on the Schedule MB/SB must be manual, from the Form 5500 instructions:
Now I have a question for you - let's say I load a PDF of a document onto my tablet, which has support for a digital pen, and I sign that document using the digital pen on the tablet. If the document requires a "manual" or "handwritten" signature, but states no requirement for ink-on-paper, does my pen-on-tablet signature satisfy that requirement?