You cannot export Salesforce Content with Data Loader. Can I export Content using DemandTools MassEffect?
Yes. Salesforce Content can be exported from the ContentVersion object. When exporting Content files, the default is to export the files in their 64base encoded format. The encoded data will be in the "VersionData" field. This is the best export method when the goal is to re-import (aka migrate) to another org.
Use Microsoft Access (.accdb) as the output file type when saving the file to avoid the encoded "body" being truncated or corrupted. Microsoft Access will use a "memo" field for this long area text field avoiding any truncation of data and any special characters will not impact the formatting of the output file.
If wanting to get the actual files exported to the PC then use the "Decode/Save as File(s)" option in the export window and specify a folder location to save the files to. The files will be decoded (from their 64base encoding) and saved locally in the specified folder.
Note: The option to decode Content files was added in DemandTools Version 2.7.
More information on how to export records using MassEffect can be found here.