PDFelement - Edit, Annotate, Fill and Sign PDF Documents
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How to Add Attachments in PDF?
PDFelement for Windows (Pro version) allows you to add attachments to PDF files. And there are two methods to add:
1) After opening your PDF file in this program, click the Attachment panel, then click the "Add Attachment File" button to browse the file that you want to attach. 
2) Add the attachment as a comment. Please click the "Comment" > "Add Attachment" button, click on the page where you want to add. An attachment symbol will appear, and you can change its style and other appearance options in the properties panel. And you can refer to this article about how to add an attachment to a PDF to get the detailed steps. 
