I am not an expert but I just want to say that In general, it's a good idea to include a cover letter or a letter of explanation (LOE) at the beginning of your PDF file when submitting documents for the Canadian Experience Class (CEC). This can help to ensure that the immigration officer who reviews your application has all the necessary information to make a decision.
In your cover letter or LOE, you can explain what documents you are submitting, why they are relevant to your application, and any additional information that you think is important for the officer to know. This can include information about jobs where you did not receive an official letter, as you mentioned in your question.
Including a cover letter or LOE can also help to organize your documents and make it easier for the officer to review your application. However, it's important to make sure that your cover letter or LOE is clear and concise, and that you do not repeat information that is already included in the other documents.
Overall, while it's not strictly required to include a cover letter or LOE with your application, it can be a helpful way to provide additional context and information that may not be immediately apparent from the other documents.