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APPENDIX H                                    Compatibility and Implementation Notes



10.10.5, “Trapping Support” (Trap Network Annotations)
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.


10.10.6, “Open Prepress Interface (OPI)”
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.


Appendix C, “Implementation Limits”
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

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