PDF causes questionable error on embedded font

[Bruggi]

The attached PDF (created with the widely used visualization library Matplotlib) leads to the following error in Adobe Acrobat. To reproduce this issue, open the PDF, navigate to Document Properties > Fonts > OK. The error message is only displayed if one changes the zoom, switches pages, etc. The error message arises in recent versions of Adobe Acrobat on different operating systems.

However, the PDF looks just fine – is there really an error and how could I get more details on what is causing this error?

[JR Boulay]

The font is not embedded in this document, you can fix this problem with Acrobat Pro (provided the missing font is installed on your computer).

[Bruggi]

Many thanks for your efforts @JR Boulay!

The font should be embedded in the PDF – where do you see that it is not?

(Is it, because there is nothing like Embedded Subset written in parenthesis behind the name of the font on your first screenshot? Unfortunately, I do not know if this notice is only shown for subsets or rather refers to embedding in general.)

According to pdffonts provided by poppler-utils the font is actually ebmedded in the uploaded PDF, as shown below.

(Poppler runs on unix-like systems, unfortunately I do not know a similar tool for Windows.)

name                     type              encoding         emb sub uni object ID
------------------------ ----------------- ---------------- --- --- --- ---------
DejaVuSans-Oblique       Type 3            Custom           yes no  no      15  0

[JR Boulay]

Acrobat Pro and PDF Studio Pro do not specify anything and Foxit Reader finds "Subset".

I believe this document must have a structural problem, but I am no longer competent.