A primary source records or documents events as seen or experienced directly by witnesses or persons involved in those events.
For more detailed explanations with examples:
Use these search terms in OneSearch to locate primary sources (search by subject and keyword)
A large-scale collaborative repository of digital content of over 10 million volumes, 2.5 million in the public doman, from research libraries including content digitized via the Google Books Project.
Eurodocs: Online sources for European History
Links connect to European primary historical documents that are transcribed, reproduced in facsimile, or translated.
World Digital Library
an international digital library operated by UNESCO and the United States Library of Congress.
The Internet History Sourcebooks Project of Fordham University is a collection of public domain and copy-permitted historical texts presented cleanly (without advertising or excessive layout) for educational use. Enlightenment Socialism
Eighteenth century collections online -- text collection partnership
A collection of 2387 works in English published in the 18th century
On the French Revolution:
French Revolution Digital Archive (official government documents from the French Revolution which have been digitalized and can be translated)
There are also examples of specific Enlightenment texts that have been digitalized:
Digitalized Encyclopedias (in English & French)
Digitalized Work by Specific Thinkers (all in French)
French printed Images: