---------- Forwarded message ----------
From: Robert Ribnitz <robert.ribnitz@gmail.com>
Date: Sat, Sep 17, 2011 at 2:00 PM
Subject: PDF Readers...
To: switzerland@fsfeurope.org


Hello there, here's a tentative list of what PDF supports, and what therefore a PDF reader must also support:

• Passwords/Encryption and digital signatures  (MD5, RC4,AES, SHA-1, SHA-256, SHA-384, SHA-512 and RIPEMD160)
• Interactive elements (checkboxes, radiobuttons), "fill-in forms" (as in text lines & text boxes)
• embedded file streams ("attachments") of any type
• ICC and DeviceN color spaces, several of the features present in Postscript Level 3
• Annotations
• Masked images, images with alternate representation, smooth shading, transparency
• Unicode charaters
• import content from one PDF into another
• PCKS #7 support with SHA-1 and RSA signaures
• Use of usage rights does not require encrypting the document
• Possibliity to hide/show sections of text, or review them
• JavaScript support
• Support for Universa 3D and OpenType
• Cross-document linking

ISO 32000-1:2008 specifies a PDF version (PDF 1.7, as supported by Acrobat Reader 8)

The following can be controlled by usage rights:

• Save the PDF, possibly with annotations, form data
  
• Import form data
• Export form data

DRM can limit copying, editng, annotations or printing; the use of DRM does not require document encryption.

see http://en.wikipedia.org/wiki/Portable_Document_Format

If we want to give users a chance of using an "alternate" reader, we must provide them with one that supports these features (or at least the more common of them).