Answer-in-theory: as many as you want
Answer-in-practise: as many as the machine can handle
In Windows, in Firefox, IE7 and IE8, you can opt to view PDFs linked from web pages in your browser. If you start opening a large number of these however, you’ll find that your browser will have a tendancy to complain…
This is due to depletion of desktop heap space. For every tab with a Reader (or Acrobat) plugin, a new Rdr/Acro process is launched, and it uses some heap space resources. So whilst your overall memory (RAM) is not running out, you’ll still find Firefox complaining at you with a dialog saying you have reached the maximum number of documents you can open.
On IE7/IE8 however, you may not see this. IE will merrily open as many PDFs as you tell it to, until it chokes, croaks and crashes. This has been recognized as an IE limitation – Rdr/Acro does not get a chance to display the dialog that prevents you from opening that one-too-many documents.
You can however do the following to prevent the crash (the usual CAVEATS concerning editing the Registry apply):
To correct this problem, increase the size of the desktop heap: Run Registry Editor (Regedt32.exe).
From the HKEY_LOCAL_MACHINE subtree, go to the following key: \System\CurrentControlSet\Control\Session Manager\SubSystems
Select the Windows value.
From the Edit menu, choose String.
Increase the SharedSection parameter.
For Windows NT:
SharedSection specifies the system and desktop heaps using the following format:
Add “,256” or “,512” after the yyyy number.
For Windows 2000, Windows XP, and Windows Server 2003:
SharedSection uses the following format to specify the system and desktop heaps:
Increase the zzz number to “256” or “512.”
With that you should start seeing the dialog pop up when you open too many PDFs.
Of course, you should also question why you are opening so many PDFs at the same time in the first place. It’s just inherently not safe.