MrMe

Thanks for this @Old Bruce, I really appreciate you taking the time. 

However, as my understanding goes (I just tested and can confirm it does act as follows) Replace Existing removes all masters from a page and replace with the selected master. This is a problem, because in some cases a page has multiple masters I don't want deleted.
For example, multiple pages in my document have Masters A, C and F.
I want them to have Masters A, C and D.
If I understand you (and the documentation) correctly, the only way I can achieve this is by removing Master F,
— either by deleting the master altogether, which then also removes it from pages I do want it on or
— deleting it manually from each page in the layers panel (which can get time consuming in a 100 page manuscript)
then adding Master D, again:
— either by apply it to all odd/even pages and deleting from the pages I don't want it on, or
— listing every single page I need it applied to.
Is my understanding of this correct? Because if it is, the available control of Master Pages still feels a little... limited.
Don't get me wrong here. I love the software, and the Master Pages feature is super useful. But I do feel that the control we have (especially in large manuscripts) could be improved.

That does solve OP's problem (and mine, hence this response), but in a much more terminal manner than I would like.
Is there no option to simply remove a specific master from all pages (or even better, a specific set of pages) without having to delete the master entirely?
My use case is as follows:
1. I have two numbering and runner masters; one for left-hand pages and one for right-hand pages
2. However, there are a number of pages that don't need the master (i.e. content title pages)
3. When I have to export a sample of the manuscript for review, it needs page numbering. However, if I needed to add an odd number of pages the masters switch to the incorrect pages (i.e. left on right). Thus, I either have to:
a: Delete the master entirely, recreate it, and reapply it or
b: Manually remove it from every page it is on before reapplying it again

I'd much rather be able to just "Remove Master From [selection] Page(s)", and then go reapply it where necessary.

Is that actually possible and I'm just missing it? If not, is there any chance that feature might become a thing in the future?
————————————
 
Adjacent to this issue, it would be excellent if we had a Master Page overview manager tool, which, on top of the current options available in the right click menu (i.e. inserting, applying, deleting etc.) would provide the following functionality:
1. Provide an overview of all master pages, and how many pages each has been used on (which also allows you to navigate to said page)
2. Allow the user to see which pages of the manuscript is using which master page (at a glance)
3. Allow the user to remove masters from specific pages without having to leave the tool
4. Allow the user to add masters to specific pages without having to leave the tool
5. Allow the user to define specific pages that should not receive any masters (even if "Apply to all/even/odd" is selected)

Is there any chance of something like that happening?

We can hover the cursor over any page in the Pages Panel and there will be a tool tip show up with the name of the applied Masters. We can see what masters are applied in the Layers Panel.
Select the pages you want to remove the master from and right-click on it, select Clear Masters. A better way is to select the pages and right click Apply Masters then choose from the options and Replace Existing.

 
  The blue pages are selected and will get changed.

Hi @MrMe,
The objects that are causing the PDF export issue are the recolour/black and white adjustments if you set rasterise option in the PDF export to Everything the PDF will export normally. It's related to the colour space of the document (32bit RGB) in combination with these adjustments, we have logged this with our developers to investigate further.
 

Just to clarify, @MrMe, it sounds like you are using the Export Persona so that you get 1 PDF file per artboard, as opposed to using File > Export which would give you 1 PDF with a page per artboard.
Note, too, that Serif is closed now for the Christmas Holiday until January 2, and it's unclear to me how many and which Serif staff might be working during that time:
 

Hi MrMe,

Would you be able to attach your document please? I'm curious about the full size of the artboards! From what I can tell is happening is that as you apply the gradient on the pixel layer, it expands the pixel layer to the whole size of the document (from the top left artboard to the bottom right artboard). If your document is quite large pixel wise it is likely struggling because of that.

To achieve the effect you want you're better off selecting the blurred pixel layer and first clicking the 'Create Mask' button. You can then draw the mask using the gradient tool onto that layer. That way the mask isn't being created too big and killing your Mac.

Thanks for the screenshot! Essentially it is trying to create a pixel layer that is 33096 x 46807pixels which is enormous and is likely using all your RAM! 
Without seeing the rest of the document and knowing the intention I can't advise on the DPI, however if it is for print (looks like it might be a photobook?) then you will want to stick with 300DPI. 
As I mentioned I will pass it on to development, as ideally the pixel layer that gets made should be constrained to the artboard size that it is being made on.
 

I've attached the screenshot — I hope this helps.
I noticed that the DPI is 300, which feels a little higher than necessary. If I reduce it to 72, will it also reduce the project weight?