Hello @Ropi27,
To find all Tabs in the current Draw document, and replace them with a Space, you could use the function Draw_ReplaceAll() specified below.
Just call it as follows  ( first open your PDF in LibreOffice Draw):
Draw_ReplaceAll( chr(9), chr(32) )
EDIT 2018-01-31 :
Or just call the helper method Draw_ReplaceAll_Tabs_with_a_Space() directly from the menu “Tools : Macros : Run Macro…”.
Sub Draw_ReplaceAll_Tabs_with_a_Space()
	Dim lCount as Long : lCount = Draw_ReplaceAll( chr(9), chr(32) )
	msgbox "Replaced " & lCount & " Tabs with Spaces."
End Sub
Function Draw_ReplaceAll( strFind As String, strReplace As String ) As Long
REM Perform a Find/Replace inside the current Draw document.
REM <strFind>	: String to be found and replaced;  (Does not support Regular Expressions).
REM <strReplace>: String to replace the found substring with.
REM Returns the number of occurrences that were Found/Replaced.
	Dim oDoc As Object  :  oDoc = ThisComponent
	If oDoc.supportsService( "com.sun.star.drawing.GenericDrawingDocument" ) Then
		Dim oDrawPages As Object  :  oDrawPages = oDoc.getDrawPages()
		Dim oDrawPage As Object
		Dim oReplace as Object
		Dim lCount As Long
		Dim i As Integer
		For i = 0 To oDrawPages.getCount() - 1		REM Traverse all DrawPages.
			oDrawPage = oDrawPages.getByIndex( i )
			oReplace  = oDrawPage.createReplaceDescriptor()
			oReplace.setSearchString( strFind )
			oReplace.setReplaceString( strReplace )
		'	oReplace.SearchCaseSensitive = True		REM TODO: Optional parameters.'
		'	oReplace.SearchBackwards = True '
		'	oReplace.SearchWords = True '
			lCount = lCount + oDrawPage.replaceAll( oReplace )
		Next i
		Draw_ReplaceAll = lCount
	End If
End Function
HTH, lib