Hi Carolina, such topic would deserve much more than a written answer.
I am happy to share some elements for your consideration:
– Apply a structured Communication & Change Management framework for the entire PMI process (starts with why, repeat the why 7 times, role of sponsors, onboard people managers, “what s in it for me”…)
– Perform when possible a “cultural gap assessment” ahead of the PMI project.The idea is to compare the culture of the acquired company with the core values of the acquiring one. Leverage similar values and protect what makes the acquisition specific.
– be clear about potential people impact (and check the HR/Legal/Workers council dependencies), especially if you create synergies or share functions.
– external communication (customers, vendors, authorities) if really needed – in case of a change of legal entity change or merger for ex.
– you may find some literature about this. I recently read this article that may give insights.