Please use GetVersionHistory to obtain a collection of VersionHistory objects for a document, select the version that is desired, and pass the VersionHistory object as the IDocumentContents argument to the export function.

Sample code:

DocumentInfo documentInfo = Document.GetDocumentInfo(repoDocumentPath, session);
if (documentInfo.IsUnderVersionControl)
{
	VersionHistory versionHistory = documentInfo.GetVersionHistory();
	foreach (DocumentVersion documentVersion in versionHistory)
	{
		string comment = documentVersion.Comment;
		if (comment.ToLower().Contains("hello"))
		{
			var docExp = new DocumentExporter();
			docExp.ExportElecDoc(documentVersion, outputFilename);
			break; // foreach
		}
	}
}