Then just get a bigger e-reader.
8" Kobo Sage is the largest I still find comfortable holding in hands for longer periods of time:
https://us.kobobooks.com/products/kobo-sage
10" Kobo Elipsa should be even better for PDFs, but I would only use it with a stand:
https://us.kobobooks.com/products/kobo-elipsa
Then just get a bigger e-reader.
8" Kobo Sage is the largest I still find comfortable holding in hands for longer periods of time:
https://us.kobobooks.com/products/kobo-sage
10" Kobo Elipsa should be even better for PDFs, but I would only use it with a stand:
https://us.kobobooks.com/products/kobo-elipsa