After added an attachment , is there any way I can use to link it in pdf?

In html, it is easy to add a link to download the attachment, but how to do it at pdf.

No, I think this may require Acrobat JavaScript which we don't support yet.

@mikeday any updates on this?

Opening an attached PDF via a link in the princexml output pdf?

No, although you can ensure that the attachments pane is open by default with this rule:

@prince-pdf {
    prince-pdf-page-mode: show-attachments
}