Learn not to be fooled by a PDF

How to tell whether a PDF is really digitally signed — A signature seal on the page doesn't mean the PDF is signed. Here's the difference, and how to check it.
How to merge signed PDFs without losing the digital signature — Merging invalidates signatures — always. But there is a way to put the documents together with every signature intact.
Digital, electronic and scanned signatures: how they differ — They're used as synonyms, but they aren't. Which one works for what, without the jargon.
Is it safe to use online PDF tools? — Most sites upload your files to their servers. Here's the risk, and how to spot the ones that don't.