That's why I was asking how you were going about it.  For example, if you set up your data link in Excel, then you can query/limit your number of people there...  and generate the letters only for those few that match the criteria.

This discussion might materially benefit from you describing how the clients are selected, rather than in generalities..
  If they have something in common ("always have FormX" or "AGI > some amount with non-taxable Pension amounts") then it can be set up as a query.
  If the particular clients are chosen manually...  yeah, there isn't a great deal that can be automated about that.