Can't highlight some PDF documents in DT3 (DTTG

I can’t annotate some PDF documents (They are imported).

Pressing the shortcut key for highlight makes a beep sound in DT3, and I can’t see Highlight in the context menu.
Interestingly, I can do so in DTTG, and also when I open them with PDF Expert in mac.

Do you have any idea?

Certain PDF documents are opened as read-only as the PDFkit might corrupt them. Is there a read-only icon in the editing/navigation bar?

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Is it a read-only icon? (I guess so)
So, I’m wondering, if it is read-only, why am I still able to annotate it with an external app and DTTG.

Most likely the external apps either don’t use the PDFkit or assume that the result will be fine.

I see. Then, how about the DTTG? Is iOS a different story?
And is there any way to solve this problem?

DEVONthink To Go uses PSPDFkit.