Q: Does it allow us to implement our own non-standard encryption layer(filter) to do decryption for reading PDFs and also encryption for writing changes like annotations or form fields?
A:
Custom security handlers are supported, as are the setting or modifying of security permissions associated with a document. Please refer to these relevant forum posts:
https://groups.google.com/forum/#!searchin/pdfnet-sdk/custom$20encryption/pdfnet-sdk/dJpyhULyYUA/0ozIje6-m5kJ
https://groups.google.com/forum/#!searchin/pdfnet-sdk/custom$20encryption/pdfnet-sdk/nJ6eZiDzUv0/mvzxA2TQIGAJ
Additionally both Android and iOS SDK come with a sample custom filters.
For Android please see:
\Samples\PDFViewCtrlDemo\src\com\pdftron\pdfnet\demo\pdfviewctrl\UserCustomFilter.java
For iOS please see:
PDFNet_iOS/Samples/CustomFilter/