The PDF Converter for SharePoint On-Premises as well as the PDF Converter API Server Platform share a common conversion engine. However, these are two separate products and licensed accordingly. The PDF Converter for SharePoint On-Premises is intended for 'on-premise' deployments and may only be used from SharePoint based environments whereas the PDF Converter API Server Platform may be used from any environment including SharePoint, Java, .NET and other web services capable systems.
The OCR and PDF/A Archiving Add-On license is an add-on that adds additional functionality to either the PDF Converter for SharePoint On-Premises or the PDF Converter API Server Platform. This functionality, e.g. PDF/A post-processing and OCR, is usually associated with more complex environments and has therefore been separated from the main product. Please note that the OCR and PDF/A Archiving Add-On is a license that must be applied alongside a valid license of the PDF Converter for SharePoint On-Premises or PDF Converter API Server Platform. A separate download of the Professional version of the software is not needed, the license unlocks all functionality.
Then there is the PDF Converter for SharePoint Online. This is a version of the PDF Converter for Office 365 / SharePoint Online and runs 'in the cloud', details can be found on this product's website. Note that the Online version can also be installed in SharePoint on-premise (2013 and later), which may be an option for some customers who do not wish to deploy anything on their own servers. For details see this blog post.
Make sure you purchase the correct product based on your needs and environment. For more details about licensing our software see this article. A high level overview of the different PDF Converters is available here.