 However, tabular data in unstructured digital documents, e.g. Portable Document Format (PDF) and images, are difficult to parse into structured machine-readable format, due to complexity and diversity in their structure and style,  especially for complex tables.
<coherence>  However, tabular data in unstructured digital documents, e.g. Portable Document Format (PDF) and images, are difficult to parse into structured machine-readable format, due to complexity and diversity in their structure and style,  especially for complex tables.
 However, due to the complexity and diversity in their structure and style, especially for complex tables.