APPENDIX H
1128
Compatibility and Implementation Notes
170. Older viewers may fail to maintain the trap network annotation’s required
position at the end of the
Annots
array.
171. Older viewers may fail to validate trap networks before printing.
172. In Acrobat 4.0, saving a PDF file with the Optimize option selected causes
a page’s trap networks to be incorrectly invalidated even if the contents of
the page has not been changed. This occurs because the new, optimized
content stream generated for the page differs from the original content
stream still referenced by the trap network annotation’s
Version
array. This
problem has been corrected in later versions of Acrobat.
173. The Acrobat 3.0 Distiller application converts OPI comments into OPI
dictionaries. When the Acrobat 3.0 viewer prints a PDF file to a PostScript
file or printer, it converts the OPI dictionary back to OPI comments.
However, the OPI information has no effect on the displayed image or
form XObject.
174. Acrobat viewer and Distiller applications earlier than version 4.0 do not
support OPI 2.0.
175. In Acrobat 3.0, the value of the
F
entry in an OPI dictionary must be a
string.
176. Acrobat viewers earlier than 5.0 use the PostScript
save
and
restore
opera-
tors rather than
gsave
and
grestore
to implement
q
and
Q
, and are subject
to a nesting limit of 12 levels.
177. In PDF versions earlier than PDF 1.6, the size of the default user space
unit is fixed at 1
⁄
72 inch. In Acrobat viewers earlier than version 4.0, the
minimum allowed page size is 72 by 72 units in default user space (1 by 1
inch); the maximum is 3240 by 3240 units (45 by 45 inches). In Acrobat
versions 5.0 and later, the minimum allowed page size is 3 by 3 units (ap-
proximately 0.04 by 0.04 inch); the maximum is 14,400 by 14,400 units
(200 by 200 inches).
Beginning with PDF 1.6, the size of the default user space unit may be set
with the
UserUnit
entry of the page dictionary. Acrobat 7.0 supports a