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There is a difference between the version number of the PDF standard used by the PDF document and the version number of Adobe Acrobat with which the PDF document is compatible. The “version of the PDF standard” is often referred to as the “PDF version” for short.

PDF version	Is supported by
PDF 1.3	Adobe Acrobat 4.x and later
PDF 1.4	Adobe Acrobat 5.x and later
PDF 1.5	Adobe Acrobat 6.x and later
PDF 1.6	Adobe Acrobat 7.x and later
PDF 1.7	Adobe Acrobat 8.x and later

If you edit a PDF document of a different version than your version of Adobe Acrobat , you may be changing the PDF version of the document without realizing it. For example, suppose you have a PDF document which was last saved in Adobe Acrobat 5.x. The PDF version of this document is 1.4. You open this PDF document in Adobe Acrobat 7.x, change it and save it. Normally, you will then change the PDF version of this document to 1.6. To prevent this, you can select the option Prevent the automatic increase of the PDF version.

Note:

You can open PDF 1.7 (Acrobat 8) documents in Adobe Acrobat 7, but you will get a warning saying that the document “may not open or display correctly”. You can edit and save these documents in Adobe Acrobat 7 and still keep their PDF version 1.7.
Exceptionally (e.g. when trying to edit and save a signed PDF using "Save as"), Acrobat does increase the version number, even if Prevent the automatic increase of the PDF version is enabled.