XLS is the oldest and most common file type. It is also the primary format for Microsoft Excel versions 97-2003. As for XLSX, it was introduced as a replacement for the XLS file type with the introduction of Excel 2007.
In terms of functionality, the difference lies in the binary streams. In other words, XLSX is based on Office Open XML, which actually stores data in XML — in a ZIP file. It also doesn't support macros and can only be opened in Excel 2007. Overall, XLSX is better optimized and takes up less storage.
When using our PDF tool, you can convert either an XLS or XLSX file to PDF — we support both. However, if you want to convert to Excel from PDF, your final file will be in XLSX format.