APPENDIX H
1098
Compatibility and Implementation Notes
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The header specifies version 1.3 or later. If the user alters and saves the docu-
ment, the viewer allows the file to be incrementally updated, since it does not
believe the version needs updating.
In both cases, the version number in the document catalog is maintained at 1.4,
and later versions of Acrobat will recognize the correct version number.
H.2 Feature Compatibility
Many PDF features are introduced simply by adding new entries to existing dic-
tionaries. Earlier versions of viewer applications do not notice the existence of
such entries and behave as if they were not there. Such new features are therefore
both forward- and backward-compatible. Likewise, adding entries not described
in the PDF specification to dictionary objects does not affect the viewers’ behav-
ior. (See Appendix E for information on how to choose key names that are com-
patible with future versions of PDF.)
In some cases, a new feature is impossible to ignore, because doing so would pre-
clude some vital operation such as viewing or printing a page. For instance, if a
page’s content stream is encoded with some new type of filter, there is no way to
view or print the page, even though the content stream (if decoded) would be
perfectly understood by the viewer. There is little choice but to give an error in
cases like these. Such new features are forward-compatible but not backward-
compatible.
In a few cases, new features are defined in a way that earlier viewer versions will
ignore, but the output will be degraded in some way without any error indication.
The most significant example of this is transparency. All of the transparency fea-
tures introduced in PDF 1.4 are defined as new entries in existing dictionaries
(including the graphics state parameter dictionary). A viewer that does not un-
derstand transparency treats transparency group XObjects as if they were opaque
form XObjects. This is a significant enough deviation from the intended behavior
that it is worth pointing out as a compatibility issue (and so is covered in imple-
mentation notes in this appendix).
If a PDF document undergoes editing by an application that does not understand
some of the features that the document uses, the occurrences of those features
may or may not survive. If a dictionary object such as an annotation is copied
into another document during a page insertion (or, beginning with Acrobat 2.0,
during a page extraction), all entries are copied. If a value is an indirect reference
to another object, that object may be copied as well, depending on the entry.