It seems like files are getting uploaded into a third party server when we are using rest API. Is there any way to do processing in our server (merge())
Hello, I’m Ron, an automated tech support bot 
While you wait for one of our customer support representatives to get back to you, please check out some of these documentation pages:
Guides:- Server side usage
- Open a document, add annotations, download - 3. Downloading the file
- Getting a processed file
Hello lakshan,
The third party server is hosted by PDFTron, here is overview of the REST API: PDF.js Express Utility API | Documentation you are unable to self host our REST API.
Best regards,
Tyler Gordon
Web Development Support Engineer
PDFTron