As a PA, I would often be a manager's first point of contact with people from both inside and outside the organisation. Tasks are likely to include:
devising/maintaining office systems, including data management and filing
arranging travel, visas and accommodation, and occasionally travelling with the manager to take notes or dictation at meetings or to provide general assistance during presentations
screening phone calls, enquiries and requests, and handling them when appropriate
meeting and greeting visitors at all levels of seniority
organising and maintaining diaries and making appointments
dealing with incoming email, faxes and post, often corresponding on behalf of the manager
carrying out background research and presenting findings
producing documents, briefing papers, reports and presentations
organising and attending meetings and ensuring the manager is well prepared for meetings
liaising with clients, suppliers and other staff.