Examining a DocumentTermMatrix in RTextTools
a documenttermmatrix is a simple_triplet_matrix. You can turn this into a simple matrix with the as.matrix command and then use all matrix functions.

# turn into simple matrix
mat &lt;- as.matrix(doc_matrix)

# vector of the words 
word_vector &lt;- colnames(mat)

# Dataframe containing words and their frequency
df_words &lt;- data.frame(words = colnames(mat), frequency = colSums(mat), row.names = NULL)