R Markdown files are examples of literate programming documents that combine R code with results and explanations. Such dynamic documents are designed to execute easily and reproduce study results. However, little is known about the executability of R Markdown files which can cause frustration among its users who intend to reuse the document. This paper presents a large-scale study on the executability of R Markdown files collected from GitHub. Results from our study show that a significant number of R Markdown files (64.95%) are not executable, even after our best efforts. To better understand the challenges, we categorize the exceptions encountered while executing the documents into different categories. Finally, we develop a classifier to determine which Markdown files are likely to be executable. Such a classifier can be utilized by search engines in their ranking that help developers to find literate programming documents as learning resources.