Just because something is a primary source doesn’t mean that it doesn’t have bias or that the facts shouldn't be verified. Start by determining the purpose/bias of the author of the document.
Here are some factors to evaluate:
If you are concerned that the author might be biased, you should consult other accounts and compare them. If several eyewitness accounts agree, you can feel confident that the events occurred as described in your original source.
Don't forget to validate/verify the information in the document. What other sources might you consult to verify the information in this item? For example, city directories and phone books can be used to confirm addresses.
Does your source support the information you found in your secondary research? If not, you should try to determine why this is the case.