Technician reported an aircraft had to return to the departure airport after the forward avionics compartment door was left open during a maintenance action; causing the aircraft to be unable to pressurize.

Narrative

While performing Integrated Drive Generator (IDG) troubleshooting on Aircraft X; I opened the E&E Bay to run a Built in Test Equipment (BITE) test on the IDG Computer unit in accordance with my reference manual around XA00-XA30. The BITE Test indicated that the #1 Engine IDG would need to be replaced. I intentionally left the E&E Bay opened due to the fact that the new IDG would require another BITE Test after installation to verify that it is in working order. After ordering all the required parts and tools for the IDG installation (Between XA45-XB00). I went to prepare and familiarize myself with the procedures on installing the IDG. After receiving the parts and tools (Between XC00-XC30) I was told that the IDG change was still a go. I informed my co-workers who were assigned to help me the process and procedures needed to accomplish the job I then proceeded to open the #1 Engine cowling and started to accomplish the IDG change. I was approached by my crew chief around XD00-XD30 that the job would be on hold as they might be able to defer the job. At XE00-XE30 I was informed that the IDG Change was going to be deferred and we need to clean up; close the engine bay; and complete our paperwork. I volunteered to clean up and close the engine bays while my co-worker return the parts and tools to their proper location. After completing paperwork I closed the flight. I regrettably do not remember ever closing the E&E Bay while cleaning up my work area. I was later informed that the E&E Bay was indeed left open and the plane was unfortunately made to do a return flight back to ZZZ to be closed. Numerous factors played a part for this occurrence. Complacency and stress played a huge part on my end; being able to be alert and retrace my steps after a job would have rectify this situation before it became a problem. Operational time pressure also played a role for the fact that rushing anyone especially with highly priority workload such as this will ultimately lead to mistakes. Also the lack of awareness from myself and other Company workers (mechanics and other departments who are task to verify that the plane is cleared for flight) failed to see the bay was left opened. Having more awareness first and foremost is something I should implement in my daily work routine. Also having better time management and realistic goals beforehand as far as management/supervision go would reduce unnecessary stress and possibly prevent situations like this from occurring.