HP 401m prints PDF files incorrectly

If it has the same driver and the others work properly consider:

1. More memory for the printer.
2. Upgrade the printer firmware (assuming one is available).

I would not even begin to think about adding memory to the printer. If the printer runs out of memory, it will tell you so. If it has sufficient memory, adding another GB will not change anything, neither the quality, nor the speed. Also, the 400 series does not take extra memory (the duplexing versions have double the memory because they work on 2 pages at once).

the format of the file is not exactly like the original

How does it differ? In what way is it different? And, what do you mean by "original"? Does that mean compared to the screen, or to something else? Are you using the driver or "Direct PDF printing"?

Posting a print sample (and an "original") would be a good idea.

To add to what @hdhondt asks: "... Are you using the driver or "Direct PDF printing"? ...":

As this printer model does appear to support Direct-PDF (v1.7) printing, try sending the content of the PDF file direct to the printer port.
If you have a M401dn or M401dw model, do this from a Windows system by using the 'lpr' command within a command-prompt session.

...or (again, if you have a M401dn or M401dw model) perhaps put the PDF file on a flash drive, and use the walk-up USB port on the printer.

Using either or both of these methods, how does the printed file look compared to what you want, and how it prints via your existing mechanism?

Unfortunately I can't answer all your questions now.  I won't be back at the location until Tuesday.   I did update the firmware last night.

I'll test next week.

I just printed the PDFs from her computer to my printer. They are identical.  Same problem.  Then I printed from my computer and the problem was not there.   Hence the problem is on her computer.  I just updated Adobe yesterday so that's not the issue.

Where do I start looking?

I still need help on this.  See my last post.

Then I printed from my computer and the problem was not there.   Hence the problem is on her computer.

You can try the following in order:

1. SFC /SCANNOW from an admin command prompt.
2. DISM.exe /Online /Cleanup-Image /Restorehealth (takes 15 - 20 minutes) from an admin command prompt for Windows 8 and above.
3. Operating system Repair Install.

Can these be done remotely?

1 and 2 can be done remotely.

Doing a repair install is different for Windows 7, Windows 8 and Windows 10.

What exactly are the differences between the good and bad prints?

It would be most useful if you could post print samples. If you can't get to the location, get the user to scan and email a print to you.

That's what I was thinking about, but it's no more than a random guess at this stage.

I can do this Tuesday.

The font was the problem.  She was using Acrobat Pro and didn't have the font the PDF was created with.