PHP securing PDF appears corrupt with Adobe Reader X

I've had a probelm turn up that is dumbfounding me. We have developed a PHP module, to take an existing PDF (PD开发者_JS百科F version 1.3) document and add a programmatically generated 'image', update the properties and secure the document. I'm using www.setasign.de's libraries (upto date - and bug fixed... )

The generated document opens fine in both Adobe and non-Adobe PDF readers, apart from Adobe X reader. The reader (X) tells the user that the document is corrupt, and won't open. We have some other X users that this works ok with - and we have byte compared the downloaded files and know the document is delivered ok.

The security is using 40bit RC4 and only allows printing, it has a master password but no user password. I've resolved this to - it works in X when security is off and doesn't work in X when security is on. I've checked the document against PDF reference 1.3 (which is what the document claims to be) and it appears to conform.

Any thoughts would be greatly appreciated, and help with my sanity.