Adobe Acrobat shows a message about an invalid page content even when this content is not broken.

It happens in the next case when there are 2 things:

1. A content of a page contain 'gs' command that is used 'TK' key.

2. The second case is very difficult to define.

Looks that it is related to the length of a stream of a page. But maybe there are exists additional conditions.

Adobe Acrobat throw the error windows when you open this document.

Note any others PDF viewers and checkers does not found any errors.

if you remove 'TK' key from 'G2' graphic state (see '17 0 obj') then the Acrobat issue also go away.

(Just rename it to any not used value, for example: replace "/TK true/" to "/TA true/")

17 0 obj

<</BG2/Default/BM/Normal/CA 1/HT/Default/SM 0.1/SMask/None/TK true/UCR2/Default/ca 1>>

17 0 obj

<</BG2/Default/BM/Normal/CA 1/HT/Default/SM 0.1/SMask/None/TA true/UCR2/Default/ca 1>>

Also if you change a value of 'Length' key for the '28 0 obj' (content of this page) to a value 2001 or more then issue go away.

Note: Yes, it is not correct to modify this value, but it is just for the test purpose.

28 0 obj

<</Filter/FlateDecode/Length 1751>>

28 0 obj

<</Filter/FlateDecode/Length 2001>>