I was mistaken it doesn't have to do with scale being a vector object in the pdf (obviously) but there seem to be some buggy behavior, not sure if its AC or the PDF. Perhaps something for @Nguyen Minh to look into.
If you open the PDF and just change the objects position in the content structure (doesn't matter where, it can even be the same position), save and update the drawing in AC it will import the scale graphic. It seems as you just need to do some changes in the structure that gets saved e.g adding text works as well.
It will remain as a vector object and can be exploded but if its just for scaling there's really no need for the scale graphic as the PDF page size is correct. Just set the viewport scale to the same as the map (1:2000) before you import and you are set.
Your navigation pane is not showing content. Right-click the pane and add content.
You have put it as a child in the image object. Just drag it so you see the placement indicator under the text object and place it at the same place it has from the start.
It's a bit wonky and I don't really have any adobe acrobat expertise so it's trail and error. But as said if you don't need the graphic as such - just set the right viewport scale befor importing and the map will be correctly scaled in AC.
The content of the given PDF has an issue: to be more precise, there is a path (this is the terminology for polygon, polyline in PDF) and some text objects nested. This is not a standard way of constructing PDF content, and this issue is handled differently in different PDF rendering software. The library used by Archicad ignores the lines, while Adobe Reader displays all of the objects. Other readers can behave the same way like Archicad, for example, embedded viewer in Firefox.
Unfortunately, this is the limitation of the PDF engine implemented in Archicad. The only workaround we can think of is manually drawing the missing lines from the PDF.
I hope this answers your question. I'm very sorry for the inconvenience and thank you very much for your understanding. Feel free to ask if you have any further questions!
Yes, it seemed likely to be caused by the PDF content structure and by extension how the PDF was created (its generated by the Swedish surveying authority).
What was strange was that AC imports the path if you save an edit to the PDF eg. adding a text box. It also imports if you moved the path in the structure, even to the same position, and save. But perhaps Adobe Reader does fix the underlying issue when saving?