Hi Lc,
Thanks for the clarification.
Currently we do not have support for inserting links that point to external pdf files on disk so I have created a feature request with the development team. You can track the live status of this issue here
http://www.syncfusion.com/support/features/pdf/Default.aspx?ToDo=view&questId=289
As a temporary workaround you can consider using one of the two approaches
1) Insert a weblink [Essential PDF supports this feature] and make it point to a file on disk. The problem with this approach is that the linked file opens inside a browser.
2) You can also embed the external file inside the main PDF document.
I have created a sample to demonstrate both workarounds
LinkToExternalFile_274.zip
Please let me know if you have any questions.
Thanks,
Stephen.