You can use a MiniDIN-8M to MiniDIN-8M Macintosh printer cable for a direct connection between the Modem Ports of the PB 140 and the SE/30. Alternatively, you could use LocalTalk or PhoneNET cabling.
In short: Start both computers. Enable AppleTalk on both machines (in the Chooser). Restart. Enable/start File Sharing on at least one machine (name and possibly password in the control panel). Select AppleShare in the Chooser (left) of the other machine. Select a computer to connect to in the Chooser (right). Click OK. Enter data in the connection dialogue, and click OK. Select a shared disk to use in the following dialogue, and click OK.
There are many possibilities, including sharing settings under the File menu. Details can be found in the System 7.1 reference handbook