My first post on here, hope it's clear:

When using ListDensityPlot, I find that the axes and tick labels are not vector graphics (i.e. they're blurry when you zoom in), when exported as a PDF. I've tried Export, Save Graphic / Selection As... -> PDF, File -> Print Selection... -> PDF, and none of them work on their own. **Is there a simple way to do this?** 

I've posted my "solution" below as an answer to this question - it might be helpful to others who have the same problem in the meantime, but maybe there is a better solution out there.

Extra points:

* Exporting as an SVG produces a terribly blurry image (no idea why). I'm really after a vector graphic, not a high-definition JPEG or PNG.
* I've tried 'save graphic as -> options -> highest quality vector resolution' (and all the other combinations) and it still produces a burry ~214kB image.
* plotting using Plot[...] and similar functions for line plots does not have this problem at all (only the known 'save as' bold font problem).
* I'm using Mathematica 11.3.0.0 on Max OS X x86.
* I'm pretty sure previous recent versions didn't have this problem.

Minimum working example:

```
xmin = 0;
xmax = 10;
xstep = 0.5;
ymin = 0;
ymax = 20;
ystep = 0.5;

data = Table[
   x^2 + y^2, {y, ymin, ymax, ystep}, {x, xmin, xmax, xstep}];

plot = 
  ListDensityPlot[data , DataRange -> {{xmin, xmax}, {ymin, ymax}}, 
   ColorFunction -> "Rainbow", 
   LabelStyle -> {20, Black, FontFamily -> "Helvetica"}]

```

Which produces this (exported as a PNG to post on here):
[![png export of plot](https://i.stack.imgur.com/O8Ujr.png)](https://i.stack.imgur.com/O8Ujr.png)

And this is a PNG screenshot of an zoomed-in, exported PDF version of the image, to show how blurry it is:

[![screenshot of zoomed pdf](https://i.stack.imgur.com/j0BGF.png)](https://i.stack.imgur.com/j0BGF.png)

Thanks in advance!