There is no legally prescribed "format" for writing a Patent Pending document. It needs only (mind the split infinitive) to "fully describe" the "new and inventive" product, composition, system, app, or steps in a method.
Typically, a combination of the marketing document, photos of the product, and a parts list is sufficient. However, also consider:
Patent Pending documents are not examined by the Patent Office. And, a Patent Pending Number will automatically be issued within a business day of filing the Patent Pending application.
The Patent Office is obliged by law to keep Patent Pending documents secret. Ultimately, the Patent Office will destroy the document without publishing it. Unless you choose voluntarily to disclose the document, its content will never become "public".
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