A few things to keep in mind when looking for and working with primary sources:
A scanned image of a document illustrates what the original document looked like (including handwriting, marginalia, etc.). This might give you a different perspective on the document than a transcription, in which the document is reduced to its text.
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.