The sentences of this article are hard for the reader to understand.
To understand relates to the reader understanding "the sentences", here. No ambiguity exists here; this is the only logical interpretation.
The sentences of this article are hard for the reader to be understood.
To be understood has some issues here. There are two possibilities of where it attaches:
- the sentences of this article - which seems to be the meaning you intend
- the reader - is the reader misunderstood? Since "to be understood" is close in the sentence to "the reader", and could describe the situation existing with "the reader", this may be how the sentence is taken by default - but this interpretation doesn't seem very sensible, so it's confusing. (For a similar but valid construct, consider Loud noises make it hard for the speaker to be understood.)
Therefore, I would go with the first version. It is perfectly grammatical, and it makes logical sense.
The sentence could also be phrased thus:
The sentences of this article are hard to understand for the reader.
The sentences of this article are hard to be understood by the reader.
Notice the prepositional change in the second rephrasing. (For doesn't seem to work here.) I still don't like this version but at least its intention is clearer.
Lastly, consider the briefest rephrasing of all:
This article's sentences are hard to understand.
This saves several words and removes all ambiguity (except, perhaps, that now I allow for listeners to have the same misinterpretation, but I don't think that harms it a bit).