when i download the purchase order as a pdf the message mentioned below appears " An internal error occurred "
You need to:
- Provide information on your edition (desktop, server, or cloud) and version number
- Post a screens hot of the error message
- Explain what you mean by “download the purchase order.” You cannot download anything from Manager. Are you referring to creating a PDF?
This is desktop edition
Screen shot attached
When i go to purchase order pdf download that error will appears
You did not attach a screen shot. Use the upload icon above the composing pane: 
And, as I said before, there is no “pdf download” function. You also did not provide the version number. If you want help, you need to follow the instructions.