Can a PDF attachment be malware?

Can a PDF attachment really be malware? I've heard that hackers can embed malicious code within PDF files that can infect my computer or steal my personal information. Is this true, and if so, how can I protect myself from such threats? Should I avoid opening any PDF attachments from unknown sources altogether, or are there certain steps I can take to ensure my safety? I'm curious to know if there are any best practices for handling PDF attachments safely.