Short answer: Yes!
Long answer: Very carefully!
Many archives have received grants to digitize materials and make them freely available online. This benefits researchers and helps other libraries save money! But, this also requires you to do an extra layer of evaluation. Make sure to ask: Does this digitized document come from a reputable source, such as a university, museum, or archives? Is this document referenced in other secondary sources?
You could try a title search in Google, or try your keywords with "primary sources." In addition, Google Books has a number of works that are digitized and freely available. To find the really old stuff, click the "Any Time" drop down just above your search results in Google Books, and then select "Custom Range."
Searching for primary sources can be really overwhelming because there are so many - that's why it's important to read your secondary sources first, and identify specific time periods, people, and events, to narrow your searches Think - what type of evidence or example do I need to prove my point or to highlight my argument?
Also keep in mind - the records kept are often the records of those in power, and may not reflect the *voices* of marginalized populations. But you can read sources against the grain, for example, census records, criminal records, etc. to get a better sense of the activities of those whose first-hand voices have not been preserved.
The library has a number of subscription-only databases that contain primary sources. These streamline searching and make your job easier as you know that these sources are credible and authentic. **But these are not the only places to find primary sources.** This is a good place to start, and some sources will only be in these subscription databases. It is good to cover your bases and try a few different places.
When searching in these databases, they are pretty good at searching for spelling variations. But, in a database like Early English Books Online, for example, searching for "Renaissance" will get you nothing, because the term Renaissance was not applied until later. You will need to put yourself *in the time* to search using words that may have actually appeared in the texts.
These are only a couple of examples of the multitude of open, digitized sources available.