Can linearization be combined with digitial signatures and PDF/A conformance?
Linearization is a feature to optimize PDF files for sequential reading. Although it is very useful in web based applications it interferes with other features such as digital signatures.
Linearization is a process which physically arranges the objects of a PDF document in a way such that the first page of a document can be displayed by just reading sequentially from the beginning of the file rather than accessing the objects from random positions. Furthermore, in a linearized file the objects are prioritized, grouped together and sorted by descending priorities.
This feature is mainly used in web environments and supported by browser plug-ins. However, if a reader does not support this feature, a linearized file can be read as if linearization was not present.
Linearization is meant for files that don't need to be updated (read-only files) and thus does not support incremental updates. This restriction has some impacts on digitally signed documents as follows:
A file that is linearized and digitally signed in one step is a perfectly valid file.
However, linearization cannot be applied to digitally signed documents as the process destroys the integrity of the signature.
Furthermore, if a digital signature is applied to an already linearized file by using an incremental update then the resulting file is not linearized anymore. This is particularly true in applications where multiple signatures at different times must be applied.
The PDF/A standard does not forbid linearization but states that conforming readers shall ignore linearization data in the file. In practical applications this provision cannot be easily implemented since linearization data is read before a reader knows that the document conforms to PDF/A. The good news, however, is that a file can be converted to PDF/A and linearized in one step.
