The PRO version doesn't have any ads. It has also some additional features (e.g. the "Inverse Resistor Calculator", improved LED resistor calculator), and more pinouts and resources. You will also get extra free space in the circuit simulator Everycircuit; if you launch Everycircuit from Electrodroid, you will get 78% more space (4x4 tile area) for your simulations.
Starting from version 3.x the Pro version also allow to sort lists of calculator/pinout/resource according to your preferences.
These are the features available only on the PRO version (list updated to vers. 4.5):
Info and formulas; Lookup Resistor color by value; Customizable LED list in LED calculator; Customizable values in LM317 calculator; Zener resistor calculator; Range Converter; RMS Converter; All Arduino boards pinout; Raspberry Pi, Pi0, Pi3 GPIO pinouts and specs; BeagleBone boards; GPIB (IEEE-488) pinout; JTAG pin-out; Character LCD pin-out; Power Over Ethernet; SD card pinout; SIM/Smart cards; Thermocouples color codes; Trailer connectors; PT100 Conversion Table; Fuses color codes; Automotive Fuses color codes; DIN47100 color coding; IP marking; World sockets and plugs; IEC sockets and plugs;
… and more features in future.