Paradise Lost is an incredibly difficult poem; even those who have read it multiple times still have trouble with certain parts, and it still takes a lot of patience (and time!) to read through it. It's difficulty is the result of a combination of factors. Oftentimes, Milton uses obsolete words that need to be glossed by an editor; when you have to look at the footnotes on the bottom of the page rather frequently, it breaks the flow of reading.
Besides strange words, Milton's poem contains literally thousands of allusions and references to other texts (Biblical, classical, scientific, you name it); sometimes you can ignore these, but more often than not you have to read the footnotes about them so you can understand the passage.
To top it all off, Milton also writes very long, complicated, Latinate sentences; there are often words left out that you have to supply, and sometimes verbs are separated from their subjects in a way that takes some getting used to. Also, there are elaborate subordinate clauses stacked on top of each other, and plenty of strange words used in strange ways just to up the ante.
But…if you can get used to Milton's style – and you can because Milton isn't so hard he can't be mastered or enjoyed – you will find that the difficulty has its own rewards, chief among them clarity and precision yoked with beauty.