I use the ABCPdf .Net module quite a bit in projects.  It’s perfect for generating and manipulating PDF files in server-side ASP.Net code.  But more than once, I’ve encountered this error and can’t remember the solution.
Finally I remembered the cause of the error.  You have to enable 32-bit applications in the IIS application pool here.
Go into IIS Manager and choose Application Pools. Find the app pool assigned to the site running ABCPdf.   Select the App Pool on the left, then click ‘Advanced Settings’ on the right side. Now change the Enable 32-bit Applications setting to True and apply.
When you’re done, stop and restart your app pool.  Now try your page again.  Hopefully that’ll fix it for you like it did me.
ABCPdf is available at www.websupergoo.com if you have need to stellar PDF manipulation in .Net.