Eunuchs' literacy and illiteracy are actually different in each dynasty, so we can't generalize them. Eunuchs are literate and illiterate, mainly depending on the role they played in a particular dynasty. For example, the eunuchs in the Tang Dynasty were called senior eunuchs, while the eunuchs in the Song Dynasty were mainly responsible for supervising the army and Tong Guan, or going to the Western Regions. The division of labor was different, which led to different knowledge of eunuchs.
The imperial edict of the emperor has undergone many changes. China separated the emperor's rights from the imperial power from a very early age. In the Qin and Han Dynasties, the prime minister did it for him, but because of this, the prime minister could pose a threat to the imperial power as long as he was a little malicious. For example, Zhao Gao of the Qin Dynasty and Prime Minister Li Si forged testamentary edicts.
Therefore, the prime minister system ceased to exist in the late Western Han Dynasty, and it was changed to a ministerial order. Later, after the reform of the Sui Dynasty, the system of three provinces and six departments was established, which fully divided the rights of the prime minister. This basic structure continued until the Qing Dynasty.
Most of the eunuchs in the Qing Dynasty were literate, such as An Dehai around Cixi, who knew both books and writing, while another eunuch, Li, was not only literate, but also accomplished in calligraphy. We can also see from many Qing dynasty dramas that there are indeed many imperial edicts conveyed by eunuchs. Therefore, the necessary condition for being a eunuch is not illiteracy.
In fact, it is also beneficial for these eunuchs to read the imperial edict, because these eunuchs are still eunuchs no matter how many imperial edicts they read, and it is impossible to bring about changes in their identity, thus avoiding the opportunity for some ministers who do not know the truth to master core secrets and have a good impression on them because of reading the imperial edict.