List<long> lstPA = new List<long>() { 2, 3 }; // the numbers can be added or removed

var exp1 = Predicate.FalseExpression<posts>();
exp1 = exp1.Or(x => x.post_author == 2);
exp1 = exp1.Or(x => x.post_author == 3);

var exp1 = Predicate.FalseExpression<posts>();
foreach (long id in lstPA)
{
    exp1 = exp1.Or(x => x.post_author == id);
}