To Export Document library and lists use "Export-SPWeb" cmdlet
For Document Library:
Export-SPWeb -Identity {Subsite Url} -ItemUrl {DocLibName} -Path {"ExportFile.cmp"} -Force -Verbose
#Example:
Export-SPWeb -Identity https://debug.to/questions -ItemUrl Sharepoint -Path "C:\Sharepoint\data.cmp" -Force -Verbose
Note: there is no "/" before the library name.
For Lists:
Export-SPWeb -Identity {Subsite Url} -ItemUrl {Lists/ListName} -Path {"ExportFile.cmp"} -Force -Verbose
#Example:
Export-SPWeb -Identity https://debug.to/questions -ItemUrl Lists/Answers -Path "C:\Answers\data.cmp" -Force -Verbose