Word said the PDF saved. On this version it may be in a folder you have never opened.

Open a document, choose Save As, pick PDF, point it at the network drive, click Save. The dialog closes without a murmur. The PDF is not there.

Microsoft has confirmed this one. It affects Microsoft 365 for Windows version 2609, builds 16.0.20430.x, and it bites when you save a PDF somewhere other than your own drive: a SharePoint library, say, or a network location. The file is not destroyed. It is written to C:\Users\yourname\AppData\Local\Microsoft\Windows\INetCache\Content.MSO under a random name such as 1f10f000, which is why nobody finds it by searching for the document title.

Microsoft's status on the issue is "Workaround", which in its own vocabulary means there is no fix available yet. Two routes get round it. Use File, then Export, then Create PDF/XPS Document, which takes a different path through the code and still lands where you tell it. Or save the PDF to your desktop first and copy it across afterwards. If you have already lost one, open that cache folder before Windows cleans it out, sort by date modified, and the stray file is usually sitting near the top.

A backup is no help against a file that never arrived, but it is the thing that covers the opposite case, the file you did save and later lost. Tendvane's one-click backup of Documents, Pictures and Desktop exists for exactly that, and takes about a minute to set going.