Hi Salil Harit,
To use a Custom widget (like syncfusion_flutter_pdfviewer) in FlutterFlow, the package must be supported on the Web platform.
Reference: https://docs.flutterflow.io/customizing-your-app/custom-functions/custom-widgets 
For the syncfusion_flutter_pdfviewer package to work properly on the Web platform, we need to add the pdf.js script reference in the web/index.html file.
Reference: https://pub.dev/packages/syncfusion_flutter_pdfviewer#web-integration
However, in FlutterFlow, we cannot add JavaScript references in the index.html file. We have raised a query in the FlutterFlow Community to find a solution for this issue.
Query: https://community.flutterflow.io/ask-the-community/post/add-scripts-in-index-html-c6eFOTqd6ncbQJo
Based on the response from a community member, we conclude that it is not possible to add JavaScript references in FlutterFlow.
Therefore, despite being supported on the Web platform, syncfusion_flutter_pdfviewer is not supported in FlutterFlow due to this limitation.
Please feel free to contact us if you require any further assistance.
Regards,
Imman Kumar P