This article describes first findings of the study of Russian official documents comprehensibility. The research material is the Corpus of Russian local documents and acts«CorRIDA» (subcorpus of healthcare domain, consisting of 617107 tokens). The study aims to identify lexical peculiarities of official documents using the method of extracting keywords, as well as to evaluate the obtained keywords using their language frequency. Analyzing the key word forms in the context of linguistic frequency, we proceeded from the idea that frequent items are easier to understand. This idea is traditionally used for lexical complexity assessing.