The diff tool in VSCode is typically used to compare differences between two text files, such as code files. It highlights the differences line by line and side by side, making it easy to see what has been added, removed, or changed.
In the context of this extension, it would allow a user to select two PDF files and see the differences between them. This could be useful in many scenarios, such as comparing different versions of a document or checking changes made by another person.
Please note that comparing PDF files might not work exactly the same way as comparing text files, due to the different nature of the file formats. The extension tries to convert the PDF files to a comparable format behind the scenes.