Researchers need to evaluate critically any primary source materials before using them, just as they do with any other information. See Evaluating Information, for general instructions on how to evaluate information in books, articles, and web sites. Some issues that apply especially to primary sources, and particularly those found on the Internet, are authenticity, accuracy and context.
A good website with digitized primary source materials will provide enough metadata for you to be able to use the information in confidence. It will indicate the source of the original document, photograph, or whatever it is, so you and other scholars can cite it properly or go back to see the original if needed. It will provide some background information that helps you understand the context, or from where the information was taken, and how complete it is. Since digitizing is a form of copying, errors can be introduced. Photographs can be altered, artworks can be inverted or otherwise distorted, lines of text can be omitted, etc. The best sites provide two versions of texts: a scanned version of the original material showing as much as possible what the item looks like, and a transcribed version if the scanned one is difficult to read (for example a handwritten document). Sticking to official sites, such as the archives that hold the item, or a government website for a government document, is one way to help avoid using a primary source that has been deliberately or mistakenly altered.
Understanding as much as possible about the creator of the primary work is important, as primary sources will have no editors, peer-review process or other evaluation done for you. The creator's bias, point of view, or reason for creating the work need to be understood and taken into consideration as do the social, geographical, historical, political and other contexts in which it was created.
Questions to consider when evaluating a primary source:
What was the creator's relationship to the event? Was he/she intimately involved? In what way? Just observing? From what vantage point?
Why and for whom was the document/photograph, etc. produced? What was the intended purpose? Why was it kept?
Does the text/layout, etc. reveal bias in how and what aspects of the topic are covered or in what was left out?
Is there anything that might have been considered unusual about the creator in the time and place in which they created the work?
For more details on evaluating primary sources see:
Primary Source Analysis Worksheet. By Colin Deinhardt and Agatha Barc, Reader Services & Instruction Librarians, Victoria University Library. November 2017. http://library.vicu.utoronto.ca/upload/seminars/primary_sources/primary_source_analysis_worksheet.pdf
A brief, clear and concise guide and checklist for evaluating primary sources.
Primary Sources on the Web: Finding, Evaluating, Using. By the American Library Association. http://www.ala.org/rusa/sections/history/resources/primarysources
A brief introduction to primary sources, evaluating primary source websites, and the sources themselves. It provides instructions and examples of how to determine who is responsible for a website, the purpose of the site, the origin of the document, and more.
NOTE: See Citing Sources Used for guides that provide instruction and examples for citing primary sources.