Search engines were first built to find HTML pages for keyword searching.  HTML pages are often rich in text and follow common conventions that are easy to index.  At first, search engine crawlers ignored relatively rare non-html files that contained little text.  As search technology matured and market pressures created a more competitive environment, the large commercial search engines began to index non-HTML file formats.

Now, search engines are adding PDF, Microsoft Application, image, video, and audio files to their search indexes. Most major search engines make it easy to do simple searches for image files.  Additionally, new, specialized search tools have been developed that focus on multimedia formats. Still the results of your searches will usually be HTML web pages, non-HTML files remain a small subset of information on most search engines.