David, this is such a problem for all of us in education and training.
In Australia, a good starting point for copyright discussions for non-lawyers like us is The Copyright Council. From their information sheets, I tentatively reached this understanding:
1. PDF copies:
The Fair Use or Fair Dealing provisions of the copyright law in your jurisdiction might help. In Australia, you are usually entitled to include limited portions of a document within your commentary on that document. That's not a license to link to a separate, complete copy of a document, but maybe there's a lawyer around who would opine on whether it might permit a link to a heavily annotated copy.
Yes. Apart from any legal obligation, attribute the source because a customer will think you more honest; because an academic will respect your scholarship; because a researcher will find your attribution useful; and because an author/creator will feel respected and valued.
3. Copyright outlives the original:
Yes. There is absolutely no question that, for example, Lloyd-Webber's right to control performances of "Cats" would still exist even if the original autograph manuscript had been destroyed and we only had digital copies.
The ethical issue is that distributing a part of someone's work may deprive them of income (e.g. direct sales of the material, advertising revenue, or brand-awareness) or increase their costs (e.g. support load, slashdotting), and does deprive them of control.
The copyright owner can give you explicit permission to do anything at all with her/his work.
Without the owner's permission, the Copyright Act in my country gives me limited rights: for example, students have entitlements to make partial copies to enable them to study the work individually; teachers can give limited access to partial copies within a closed class; the public has a limited entitlement to parody or reproduce part of the work in order to express an opinion; and copyright expires after a (very long) time. The limits on these are complicated, involving both a schedule in the Act and interpretations yet to be tested/made in future court cases.
Can you use the Internet Archive, and let them sort out copyright compliance?
5. Business risk
Which is worse? (a) Your client fails to reach an external resource that you have recommended by linking; or (b) You receive legally enforceable urgent take-down orders for copies that you are distributing. Consider how your use of resources differs from the ethical, legal and business issues in the Megaupload case. You do not want to be distributing a copy of any source document when the original is removed under (threat of) court order. Copyright is not the only issue. Courts have ordered takedowns due to (claims of) defamation, obscenity, privacy, intimidation, vilification, security, blasphemy, holocaust denial, among other things, and some of these determinations involve defendants operating in foreign jurisdictions.
Copyright law is "bad law" in the sense that it is virtually impossible for most citizens to understand and observe. Of course, that doesn't remove our duty to comply. I don't know that there is a simple solution to this, apart from using material in the way that the copyright owners prefer.
I'm looking forward to hearing better answers in this discussion.