A few things to keep in mind when looking for and working with primary sources:
Context - In what context was the creator of this source working? What might have influenced them?
Audience - Who was the intended audience for this source?
Format - Consider how the format in which you've found the document might impact your analysis. (Often, sources found online will be scanned and/or transcribed.)
Primary sources are firsthand accounts of events or conditions during a particular period, often recorded contemporaneously by participants or observers.
Primary sources are written in the language of the time, not necessarily the language we use now. Choose your search terms wisely, or else you might miss out on the perfect source.