Short answer: it’s both ethical IMO and fair use under copyright law (US and AUS) because the OP owns a purchased copy, and it meets the additional US requirement that the duplication involve displacement to another medium for personal consumption, which has been ruled ‘fair use’.
I don't think it's correct to automatically refer to copying copyrighted work as ‘illegal,’ ‘theft’ or ‘piracy’. There are many instances of permissible fair use of copies. I don’t believe it’s governed by criminal law either. It is, rather, ‘potential violation of copyright’, which is a commercial matter. It is also potentially completely ethical and legal ‘fair use’ depending on the circumstances, principles for which have been clearly ennumerated.
Copyright exists to encourage creativity by rewarding creators with exclusive rights to proceeds of sales, including sale of that copyright, just as patents exist to encourage innovation. Considerations of ethics are implicit in the creation of copyright and patent law, but they are overtly framed in notions of encouragement of creativity, which fosters development of things for the good of society overall.
So, there are actually two issues involved, ethics and legality. There are various criteria for judging the latter, but the crux for this instance is that noncommercial use is often fair, particularly because it does not damage the work’s potential market value or commercial value, because the individual has already remunerated the creator (or owner of the copyright if they purchased the rights from the creator) for the work. Furthermore, it is fair use because the copying is in the form of ‘displacement’ (for instance, if you scanned it into electronic form so you could read it on your tablet while traveling; or if you recorded a show to watch it at a later time slot; or made a cassette compilation or backup copy in case of damage), and such copying has been ruled fair use in the US. Under Australian law I don’t believe displacement is even a requirement. If you own it, you may make a single copy of it if you also keep the original. If you go on to sell the original you're required to destroy your copy.
Functionally, downloading a copy isn’t different from making a copy, and I don’t know how US judges would rule, but under the principle of not displacing sales, the OP’s downloading of a pdf is probably legally defensible in the US if he owns and does not dispose of, gift, trade or sell the original, and the download was free. I’ve not yet actually seen case law on this particular type of ‘copying’. I’m not a lawyer but have edited material on topics like this for a couple decades.
Ethically, I judge it as fully ethical. It’s also ethical and legal to lend someone an owned copy of e.g. a book or magazine. It’s not considered fair use under US law if you give them a photocopy or scan of the entire thing, and I think it would be in a gray area were it a partial copy and *not* technically fair use if it displaced sales, but that doesn’t make it criminal and does not necessarily make it unethical, because it is a question of copyright violation, not criminal law. Ethically I have no problem with sending someone a photocopied article out of a magazine because it’s equivalent to lending them your magazine (both displace sales but are minor acts), but I'm under the impression that the US Copyright Act and case law fail to recognize this equivalence.
As an aside, relevant to a later post above, IMO it cannot possibly displace sales if back issues are not available for purchase or a still-under-copyright book were out of print (so that *might* fly, legally, in the US). Also, seeing how good the articles from that magazine are may encourage you to get a subscription. As to the latter, I don’t think it’s technically included in fair use in the US, but if back issues were unavailable AND you went on to actually get a subscription, I think you might even be able to make a decent legal defense in the US, as the lending actually did the opposite of displacing market value.
But I don’t see it as unethical. To the extent that the creator or owner is small in scale and relies on every sale, the ethical defensibility crumbles IMO, and certainly so for secrets and inventions of magic, which cannot be ‘partially copied’, but I don’t think the US allows differentiation based on scale, because it judges it as unfair use if, hypothetically, it would displace sales were the practice to become widespread.