If you properly checked and requested runtime permissions for external in your code and your target SDK is 23 and higher in manifest, this is the code you need to copy the file from assets to a folder named Download in the default external:
File.Copy(File.DirAssets,"christmastree.pdf",rp.GetSafeDirDefaultExternal("Download"),"christmastree.pdf")
You should find the file under: /Android/Data/your package name/files/Download