When launched within a sandbox, NP7 reverts to trial.
To avoid you have to check to proper software compatibility rule within sandboxie.

As of today, there are only rule to Nitro PDF 5 & 6.

As far as I can say, enabling Nitro PDF 6 compatibility rule will work with Nitro PDF 7.
Add in your global settings or in your per-sandbox settings.

The only flaw there is is that SBIE can't detect by itself you use NP7... Maybe if Tzuk comes over here, he will update the compatibility rule, but at least, everyone has a working solution.

From looking at an install of the trial version 7, it looks like they have changed the name of the service.

Software Compatibility looks for:
NitroDriverReadSpool

but version 7 uses:
NitroDriverReadSpool2

I don't know what Windows version Franck uses, but on XP the service name can be verified with:
Start > Run > services.msc > OK
Scroll down to the display name that starts with:
NitroPDFDriverCreatorReadSpool
(it will probably say "NitroPDFDriverCreatorReadSpool2")
Double-click on that entry and look near the top for "Service name:"

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Unfortunately, I don't have a suitable virtual machine setup to see if anything else in the template might need to be changed.
And I'm unlikely to upgrade my V6 program to V7, at a $50 upgrade price.

You're right the service name changed.
I'm using win 7 64 bits, but I looked for the service name in winpatrol. I read NitroPDFDriverReadSpool2. Not much in common with NitroDriverReadSpool
Now, using the rule for Nitro 6 is OK to me... but...

1/ When I open a pdf from the web browser, NP7 launches in the sandbox as expected.

2/ When I create a pdf using the nitro pdf printer (whose name was also append with this ugly "2") :
- all the pdf generation occurs outside the sandbox
- NP7 loads and opens the new file outside the sandbox.

Ok to me, but better to be aware. And hopefully, a template for NP7 will be added some day.

Prior, I was using NP5 I had for free during a special. Today there was 40% off the regular price at http://www.bitsdujour.com/software/nitro-pdf so I decided it was time to become a paying customer.
But from v6 to v7, except maybe for ocr, I'm not sure it is worth the upgrade price...

Just a point of clarification ...
"NitroPDFDriverReadSpool2" is the "Display name" for the service.
"NitroDriverReadSpool2" is the "Service name", and the service name is what Sandboxie's Software Compatibility looks at.

When the NP7 program adds a "pdf printer" to Windows, it appears on the list of printers that can be used by any program that can output a printed page.
Sandboxie allows sandboxed programs to send data to printers using the Windows print spooler, which is a part of Windows and is running unsandboxed.
That's necessary if a sandboxed program is going to be able to print documents using Windows.

The data that is output to this "printer" is formatted for use by a printer, so it doesn't represent any security risk as far as outputting malware to the unsandboxed system. For example, when a program writes to this printer it can't output executable program code to the printer, but will only send data that is formatted for use by a printer. The pdf printer then generates a file from the printer data that was sent to it.

If NP7 is running sandboxed, and the new file is generated in a location that is outside of the sandbox, and if NP7 is set to open the new document after it's generated: then isn't NP7 still running sandboxed, and is now opening a file that happens to be outside of the sandbox?
If so, then you are still opening the file with a sandboxed program, just the same as if you started NP7 sandboxed and had it open a file.
And since it's extremely unlikely that there could be any malware in the data that NP7 sent to a "printer", then the file that was generated outside of the sandbox is safe too.

OK, now I realized why my nitro PDF is not functioning that good. I see that upgrade costs $50. Do you think, if I wait a little longer, will the price go down? Or, is there any other options? Just wondering though.

If you mean when it is running sandboxed, then what version of Nitro PDF do you use?
Version 5 and 6 both worked well for me when sandboxed, and each of those Nitro versions has it's own template to use with Sandboxie.
The correct template should show a [+] in the listing at:
Sandboxie Control menu > Configure > Software Compatibility
Either "Nitro PDF 5" or "Nitro PDF 6"I don't know, but I don't use the program enough to pay for whatever features they may have added.

In version 3.63.06, I adjusted the Nitro PDF template to detect the new service NitroDriverReadSpool2 as discussed in this topic.