A PDF may have dependencies on external files.
Any external files that are referenced by the PDF may change or disappear, and this may affect its rendering.
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.
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:
|Reference file||Description||Error Code(s)||Details|
|externalLink.pdf||Contains link to another document||6.2.5||Action is forbidden, The action Launch is forbidden|
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.
- http://www.opf-labs.org/format-corpus/pdfCabinetOfHorrors/ - PDF Cabinet of Horrors on OPF Format Corpus
- PDF/A Allows External Hyperlinks - White Paper, EBriefPro, Inc.