"如此" here emphasis the significant difference among Chinese characters. It's equal to 'so' in the sentence below: "You are so beautiful!"——你是如此美丽！ "以及" means 'and' here. Translate the original sentence to English may make it easier: "They(the Chinese characters) are so different and hard to understand."——它们是如此的不同以及不可理解。

如此 = like this but in your context, "like so" 以及 = "and so" in your context. Other meanings are "as well as, too" My English translation, "On first sight, the Chinese characters look so different that it is difficult to understand"