The phrase "to illuminate any manuscript" means to enhance the appearance of a written document by adding decorative elements such as colorful designs, illustrations, or intricate patterns. Full definition
Each segment of the border is filled with detailed illustrations, contributing to the illusion of the book as a medieval illuminated manuscript. (booklistonline.com)
The book is a rare chance to get intimate with twelve illuminated manuscripts from the medieval period, which you are not likely to get to see in real life. (inspiringinterns.com)
I am no scholar of 13th or 14th century illuminated manuscripts. (writerunboxed.com)