When I store PDF in the Brain, I can read then, but I can't hightlight text or add notes in the PDF. Is that possible ? If not, it should be, imo, a new feature, soon.
The PDF application Itself, and not TheBrain, really controls what you can or cannot do within the PDF when you open it.  There are  PDF applications that permit adding notes, editing, highlighting, etc. For example, within Windows 10, You can purchase a Win 10 app for $11.99 called DrawboardPDF which provides a considerable amount of drawing and commenting capability. The high end of professional  PDF  applications is Adobe Acrobat DC which is highly functional in terms of what you can do to the contents of a PDF.

If you are speaking of why you cannot edit or insert notes in the TB Notes view of the PDF,  that is not possible in the present functionality of TB10 as  you are simply viewing an image presented by TB. As I think you are suggesting, possibly the TB Development team could somehow provide this functionality in the future. Note - you could always add your notes below the PDF image I believe.
customized Desktop PC (Haswell i7 4770 3.4 gHz with 32 GB RAM), Windows 10 Pro 64-bit, 256 GB SSD,  and a Microsoft Surface Pro 6, Windows 10, 512 GB SSD, 16 GB ram, i5

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