The warning came in a memo from chief press secretary Bernard Ingham setting out her strengths and weaknesses.
The 1985 memo was released by the Margaret Thatcher Archive Trust, in charge of the gradual release of her private files.
Ingham's five-page memo to Thatcher states that she had gained a reputation among the public of being "hectoring, strident and bossy".
On the positive side, he described her as a "decisive, strong-minded person" and "someone who is clearly going to be very hard to beat".
"In short, you remain the dominant person in British politics," he added.
Ingham tried to get her to change her style for the Conservative Party conference in Blackpool but she declined to use words such as "compassion" and "caring" in her speech.
Chris Collins, from the trust, said: "Her public image was so fixed that she couldn't win - if she had suddenly shown a softer side, people would not have believed it."
Other papers from 1985 contain reports of a Commons "scuffle" between Labour MP Robert Kilroy-Silk and future party leader Jeremy Corbyn.
Plans to exclude Thatcher from the unveiling of a Falklands War memorial at St Paul's Cathedral also feature.
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