while I do not know if the PDF book is protected by copyright or not?

I can't help pointing out that it would be an awfully good plan to ... drumroll ... find out. Is there a date anywhere in the book? Does it have a named author? Do these dates place it in the public domain in the country where its server is located? (Did Canada end up getting dragooned into extending from 50 to 70? I know Canada does a lot of stuff at the provincial level-- even some things that are federal in the US-- but copyright was still national the last time I looked.) If no, don't bother linking; it might disappear at any time.

In general you will not get in trouble for actions or statements in the form "It is illegal to do X here, but you can lawfully do it in jurisdiction Y". (There are plenty of benign examples.) Unless you live in a weird jurisdiction, in which case it is safer to consult a local attorney than ask in an international forum.

ça depend(s)*..it depends..

*could be plural, could be singular..depends ;) on the context..

If the query is a direct link to a pdf.... then know if the pdf is suitable, ie. there are books in copyright which have license to share, they are not in public domain, but the copyright holder allows distribution.

There are pdfs which are in copyright and require payment. These are not allowed for public distribution.

There are pdfs that are not to be shared no matter what (though why any idiot puts them on the web we shall never know).

If you provide a link to a page that has links to such pdfs, then it is less likely you can get in trouble, but is no guarantee against prosecution.

The PDF book has no date. There is an author name. I know that the author is dead but I'm not able to find when he died. I can not find any information on the copyright of the book...

I think I'll just summarize this book. So I'll be sure to respect the copyright laws of all jurisdictions on the planet... ;) Better safe than sorry.

Lucy24 and Tangor... thank you very much for your very pertinent comments. :)

Leosghost... thank you for your esprit français. :)

Surely the book has some existence other than the lone PDF that you've found. Have you tried an exact-text search of a few randomly selected phrases? What with Google Books trying with might and main to become WorldCat, you can find just about everything there. (They seem to be working backward; the closer a text is to 1922, the more likely it is to have been scanned.)

Minor clarification on what I said earlier: If you have to ask you probably already know that link might not be good. Why? Because you asked. :)

On the other hand, there are PDF's any can link to from their sites if open distribution is allowed. Determine that before you do it.

@lucy24 --- there are many pdfs out there which have nothing to do with google's overreach to digitize older or orphaned texts... or self-published with google exclusions.... or tech manuals for the latest Whiz Green Widget... that latter is copyrighted when the Widget was released.

Dead authors are, sadly, an every day occurrence. Find out the date they died to help get a handle on when the text might be available. Under the Bern Convention that is 75 years after the author passed away. (Don't hold your breath!)

Thank you for your insight! :)

It's also worth noting that if the person with the PDF has to take it down, or it's taken down, for whatever reason, you may end up with a broken link.

Engine... Yes, that's true.

Anyway, as Tangor said, if I ask ... it's because I strongly suspect it's illegal... I guess subconsciously I wanted to hear a response like this: "It is not immoral and it is not illegal to link to illegal content" ... LOL

Since visitors to my website are French speaking, I'll do a summary in French and so I'll be sure to respect the laws... and I'll be sure that my summary will not disappear!

Thank you very much for your good advice. :)

Best of all solutions! You have new content on your website, your visitors are served, and you can sleep at night. What's not to like about all that?