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APPENDIX H                                     Compatibility and Implementation Notes



The minor version number is incremented if the changes do not prevent existing
viewer applications from continuing to work, such as the addition of new page
description operators. The version number does not change at all if PDF changes
in a way that existing viewer applications are unlikely to detect. Such changes
might include the addition of private data, such as additional entries in the docu-
ment catalog, that can be gracefully ignored by applications that do not under-
stand it.

The header in the first line of a PDF file specifies a PDF version (see Section 3.4.1,
“File Header”). In PDF 1.4, a PDF version can also be specified in the Version en-
try of the document catalog, essentially updating the version associated with the
file by overriding the one specified in the file header (see Section 3.6.1, “Docu-
ment Catalog”). As described in the following paragraphs, the viewer applica-
tion’s behavior upon opening or saving a document depends on what it perceives
to be the document’s PDF version (compared to the viewer’s native file format—
for example, PDF 1.3 for Acrobat 4.0—which is also referred to as the viewer’s
PDF version). Viewers that are not PDF 1.4–aware may perceive the document’s
version incorrectly, because they look for it only in the PDF file’s header and do
not see the version (if any) specified in the document catalog.

An Acrobat viewer attempts to read any PDF file, even if the file’s version is more
recent than that of the viewer. It reads without errors any file that does not re-
quire a plug-in extension, even if the file’s version is older than the viewer’s. Some
documents may require a plug-in to display an annotation, follow a link, or exe-
cute an action. Viewer behavior in this situation is described in Section H.3, “Im-
plementation Notes.” However, a plug-in is never required to display the contents
of a page.

If a viewer application opens a document with a major version number newer
than it expects, it warns the user that it is unlikely to be able to read the document
successfully and that the user cannot change or save the document. At the first er-
ror related to document processing, the viewer notifies the user that an error has
occurred but that no further errors will be reported. (Some errors are always re-
ported, including file I/O errors, extension loading errors, out-of-memory errors,
and notifications that a command has failed.) Processing continues if possible.
Acrobat does not permit a document that has a newer-than-expected major ver-
sion number to be inserted into another document.

If a viewer application opens a document that has a minor version number newer
than it expects, it notifies the user that the document may contain information

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