References to external files

h2. Description
A PDF may have dependencies on external files.

h2. Risks
Any external files that are referenced by the PDF may change or disappear, and this may affect its rendering.

h2. PDF/A and references to external files
Contrary to some (incorrect) popular belief, the PDF/A standards do not rule out references to external files completely. The following mechanisms for referring to external content are allowed:

* _URI_ actions: these refer to Internet resources (i.e. a clickable hyperlink)
* _GoToR_ actions: these refer to an external _PDF_ file (i.e. a clickable link to a locally stored _PDF_)

Consequently neither _URI_ actions nor _GoToR_ actions are reported by Apache Preflight. From a preservation point of view, _GoToR_ actions may nevertheless be a risk (e.g. in case of a collection of _PDFs_ that refer to each other), even though the rendering of the files that contain the reference is not affected. More details on this can be found in this [White Paper|http://www.ebriefpro.com/pdfs/pdfa.pdf].

h2. Assessment
The following table shows the relevant output of _Apache Preflight_ (part of [Apache PDFBox]) for both a PDF that contains a link to another document, and a PDF that imports text from a website via the "web capture" feature. Results obtained with _Preflight_ 2.0.0:

h3. Error code 6.2.5 is generic
Error code 6.2.5 is a generic error code for any action that is forbidden in PDF/A-1, so we need to take into account the elaborate error description (i.e. the contents of the _details_ field in the output) in order to narrow things down to the _Launch_ action.


h2. Example files
* [http://www.opf-labs.org/format-corpus/pdfCabinetOfHorrors/] - PDF Cabinet of Horrors on OPF Format Corpus


h2. References
* [PDF/A Allows External Hyperlinks|http://www.ebriefpro.com/pdfs/pdfa.pdf] - White Paper, EBriefPro, Inc.