It's the way Japanese used to be written.
Hiragana being the favoured way of writing is relatively recent. Pre-war, they use katakana instead.
They also had a spelling reform after the war, so you'll find that in stuff written before that certain words are written with kana different to those they are pronounced with.
For example, verbs ending in う were written with フ (still pronounced as う, as they are today). These verbs were then conjugated accordingly, so for example 違フ, 違ハナイ, 違ヒマス, but all were still pronounces as they are today. Additionally, a lot of words ending in -ou were written as ending in -au (still pronounced -ou, as they are today), so for example そう was written サウ, 行こう was written 行カウ, and so on. And certain words were written with now defunct characters, for example いる was written ヰル.
They were written like this because a long time ago they used to have pronunciations matching those characters, but over time the pronunciations changed but the spellings did not.
After the reforms, all of these words where changed to be written how they are pronounced.
You'll find this document uses a lot of old-fashioned literary language as well. Ways of speaking that were defunct even back then in spoken language, but still used somewhat in a literary context.
A lot of them revolve around the old conjugations of the verb する, which used to be す. For example, セラル means the same as される. It uses the old adjective particles, as well, ナル and タル instead of な.
I don't know enough of the old language to be able to translate it for you, though.