Previous Next

                                         993
APPENDIX C                                                     Implementation Limits



Acrobat has some additional architectural limits:

• Thumbnail images may be no larger than 106 by 106 samples, and should be
  created at one-eighth scale for 8.5-by-11-inch and A4-size pages.
• The minimum allowed page size is 3 by 3 units in default user space; the maxi-
  mum is 14,400 by 14,400 units. In versions of PDF earlier than 1.6 (Acrobat
  7.0), the size of the default user space unit was fixed at 1 ⁄ 72 inch, yielding a
  minimum of approximately 0.04 by 0.04 inch and a maximum of 200 by 200
  inches. Beginning with PDF 1.6, the size of the unit may be set on a page-by-
  page basis; the default remains at 1/72 inch. (See implementation note 177 in
  Appendix H.)
• The magnification factor of a view is constrained to be between approximately
  8 percent and 6400 percent. These limits are not fixed; they vary with the size of
  the page being displayed, as well as with the size of the pages previously viewed
  within the file.
• When Acrobat reads a PDF file with a damaged or missing cross-reference
  table, it attempts to rebuild the table by scanning all the objects in the file.
  However, the generation numbers of deleted entries are lost if the cross-
  reference table is missing or severely damaged. Reconstruction fails if any ob-
  ject identifiers do not appear at the start of a line or if the endobj keyword does
  not appear at the start of a line. Also, reconstruction fails if a stream contains a
  line beginning with the word endstream, aside from the required endstream
  that delimits the end of the stream.

Memory limits cannot be characterized as precisely as architectural limits because
the amount of available memory and the ways in which it is allocated vary from
one product to another. Memory is automatically reallocated from one use to an-
other when necessary: when more memory is needed for a particular purpose, it
can be taken from memory allocated to another purpose if that memory is cur-
rently unused or its use is nonessential (a cache, for example). Also, data is often
saved to a temporary file when memory is limited. Because of this behavior, it is
not possible to state limits for such items as the number of pages in a document,
number of text annotations or hypertext links on a page, number of graphics ob-
jects on a page, or number of fonts on a page or in a document.

Previous Next