Why is pneumonia as a flu complication more likely to happen to people recovering from a pulmonary embolism? would you explain the mechanism please?

Not necessarily : They sometimes with or without the proper test can be confused with each other especially with doing just a Chest Xray.The both cause changes on imaging and they both cause shortness of breath.A Cat scan should be able to distinguish between the two.However,a PE causes damages to the lung which may be prone to Pneumonia.Also either can cause prolonged bed rest and can lead to the other.