If you have any doubt, at all, about whether a document is subject to copyright, then the safest way to use the document is to get permission from the copyright owner. The only way to establish a level of security in the use of another's works is to become familiar with the basics of the copyright law in your country of interest. A copyright interest is not absolute, the right is subject to several limitations. The limitations to a claim of copyright include one very important limitation: fair use.
The doctrine of fair use is codified in Title 17 United States Code, Section 107. Here is Section 107:
From a circular published by the U.S. Copyright Office, "The distinction between fair use and infringement may be unclear and not easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission"The circular goes on to give examples of activities that courts have found to be fair use:
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
...quotation of excerpts in a review or criticism for purposes of illustration or comment; quotation of short passages in a scholarly or technical work, for illustration or clarification of the author’s observations; use in a parody of some of the content of the work parodied; summary of an address or article, with brief quotations, in a news report; reproduction by a library of a portion of a work to replace part of a damaged copy; reproduction by a teacher or student of a small part of a work to illustrate a lesson; reproduction of a work in legislative or judicial proceedings or reports; incidental and fortuitous reproduction, in a newsreel or broadcast, of a work located in the scene of an event being reported.Let's say I found a really interesting article on researching probate records, so I copy the article, entirely, and reproduce it in one of my blog posts. Is this fair use? Looking at the definition, I note that copyright prohibits the use of substantially all of a document. So it appears, that my use of the article on probate records is a violation of the original author's copyright. Deciding what is or is not a substantial portion of the work, depends entirely on the ratio of the size of the copy compared to the entire document. Use of the entire document, even if the document is rather short, constitutes an infringement of the original copyright. Citation of source is not a substitute for permission to use the material.
Another example, let's say I find a history of a county published in the 1870s in a journal article. Can I quote the entire history in my book on my ancestor? The answer is going to depend on where you got the information. If you got the original source and it was outside of copyright protection, then you would be safe in using the article. But from the standpoint of honesty and intellectual integrity, you must still acknowledge your source. Just because a work is not covered by copyright law, you do not have the right to appropriate the work as your own. Likewise, if you quote the history from the journal article, you may still violate copyright if the article where the information came from was subject to copyright and you use a substantial portion of the article.
Could I copy an old photograph or design and sell it under my own copyright? Normally, unless a substantial amount of additional work goes into a work that is out of copyright, reproduction of the work does not create a new work subject to copyright.
There is a whole lot more to this subject. Next time, public domain and the genealogist.