Most of my experience with regards to text extraction is in the context of parsing and extracting data and metadata from files produced in scientific experiments. With the exception of xls documents, these are custom formats for which I would create compilers to extract and transform what text I wanted.
in reply to Re^2: (OT) question about clustering
in thread (OT) question about clustering
File formats are little computer languages in disguise. So the general approach of creating a compiler from the format you start with to the format you want will always work in general. In practice, writing compilers for each format can be an arduous process made difficult by incomplete file format specifications, eg, .doc format.
In your case, you are ETL'ing standard, albeit very different formats. If I were you, I would take advantage of the programs that create these formats to do the extraction. Use Microsoft Word to convert .docs into plain text format. Use Excel to convert .xls files to CSV files. These can be scripted easily enough using VBA/Visual Basic/Visual C# and will extract all the text there is to extract in documents. From there, it is easy to write perl grograms to transform the resulting text to your custom needs.