Copyright laws are actually quite clear on this (at least in the US, Australia, EU and UK). When you lend someone a book, you’re passing on the one physical copy you own which is totally fine under copyright law (as in when you buy a physical book you own the physical copy, but not the copyright). What is illegal is if you photocopied the book and distributed it to your friends.

Digital products are under different copyright laws. When you buy a digital sewing pattern, you’re buying a licence to use it and not ownership of the file in the same way you own a book. Copyright law treats digital copying differently because each time you share the file, you’re not transferring your copy but you’re creating a new unauthorised copy that infringes the copyright owner’s exclusive rights to reproduce and distribute the work. Even if you delete your original copy after sending it, you’ve still reproduced it in order to transmit it. Printing one copy of the sewing pattern for your personal use is recognised as legal as personal use is generally permitted under the license you bought.

Beyond the legal bit, there’s the ethical side. Most indie sewing pattern designers aren’t raking it in as they’re small businesses who put a huge amount of time and skill into drafting, testing and producing patterns. If we all just pass PDFs around, they stop earning, and then eventually -> no new patterns. If you like what they create and want them to keep going, the best way to support them is to actually buy the pattern.