In my view, it's very difficult, perhaps impossible, to find a truly reliable translation tool for any pair of languages. Any automatic translator will inevitably make word, phrase, and grammar errors. However, it is possible to find a translation engine that gives you the basic sense of what the original text is saying.
For Polish, I've been happy with Poltran. To translate a document, you would copy a block of text -- which can be thousands of words -- and paste it into the box here.