A pronoun is a word that replaces a noun. The noun that has been replaced is referred to as the antecedent for that pronoun. Pronouns must agree with their antecedents in number. A singular antecedent needs a singular pronoun and a plural antecedent needs a plural pronoun.
Indefinite pronouns are singular and must be replaced with singular pronouns, not plural. Using plural pronouns with indefinite pronouns is a common mistake. Because indefinite pronouns are singular and require singular pronouns as replacements, the indefinite pronouns often create a problem with sexist language. To avoid the problem writers often rewrite their sentences to use plural antecedents so they can use plural pronouns. In most cases, collective pronouns are also singular and require singular pronouns.
Pronouns require a clear antecedent to refer to. Writing that leaves the antecedent for a pronoun vague is often confusing and difficult to follow. It can also result in unintentional comedy on the part of the writer when the antecedents for the unclear pronouns are misinterpreted. It is crucial during the editing process for you to check each pronoun and connect it to its antecedent to be certain that the antecedents are clear and that there is agreement between the antecedent and the pronoun.