Export-SPWeb does not include folders for document library

I need to copy a SharePoint 2013 Document Libarary (which contains PDF files more than 500M bytes) from one site to another in the same SharePiont application.

The Powershell command 'Export-SPWeb' and 'Import-SPWeb' can export the content to a temp CMP file, and then import to the new site.

Example of my PS script:

Export-SPWeb http://server:1780/en/wikisite/ -ItemUrl "My Wiki Documents" -Path "c:\temp\pdfs.cmp"
Import-SPWeb http://server:1780/NewWiki/ -Path "c:\temp\pdfs.cmp"

However, if the Document Libary contains sub-folders, the sub-folder and the PDF files in  the sub-foldersdoes not import to the new document library.

Can anyone helps? Thanks!

HXMCN