Quick answer
First, save the PDF to your desktop and open it from there — if that works, the file is fine and the problem is Outlook's temp folder or Adobe. Then clear the Outlook secure temp folder, update and repair Adobe Acrobat/Reader, and turn off Adobe's Protected Mode. One of those fixes it almost every time.
WHY IT HAPPENS
Outlook and Adobe are tripping over each other.
When you open an attachment, Outlook first copies it to a hidden secure temp folder and hands it to Adobe. If that folder is cluttered or has permission issues, or Adobe's Protected Mode sandbox misfires, or the Adobe install is damaged, you get the "memory could not be read" crash — even though the PDF itself is perfectly good.
THE FIX
Work through these in order.
- Save the file out, then open itSave the attachment to your Desktop and double-click it there. If it opens, the file is fine — carry on to fix Outlook/Adobe. If it also fails from the desktop, the PDF itself may be damaged; ask the sender to resend it.
- Update Adobe Acrobat / ReaderOpen Adobe, go to Help → Check for updates, install everything and restart. Many "memory could not be read" crashes are fixed by simply getting Adobe current.
- Repair the Adobe installationIn Adobe, choose Help → Repair installation (or repair it from Windows Settings → Installed apps). This rebuilds damaged components without losing your settings.
- Turn off Adobe Protected ModeIn Adobe, go to Edit → Preferences → Security (Enhanced) and untick Enable Protected Mode at startup. Restart Adobe and try the attachment again.
- Clear the Outlook secure temp folderPress Windows + R, paste %localappdata%\Microsoft\Windows\INetCache\Content.Outlook and press Enter (open the folder inside it) — then delete the files in that folder. Outlook recreates it fresh. This clears the most common cause.
- Test with a different PDF viewerTemporarily set Microsoft Edge as your default PDF app (Settings → Default apps) and open the attachment. If it opens in Edge, you've confirmed the issue is Adobe-specific.
- Repair Office as a last resortIn Windows Settings → Installed apps, find Microsoft 365 / Office and run a Quick Repair, then restart and test again.
Happening across several PCs? Then it's not one person's machine — it's usually an Adobe or Office update that landed badly, or a group policy. That's the point to hand it to your IT provider to fix at the source, rather than repeating these steps on every desk.
