RESOLVED: To read PDFs, must install Ghostscript -- credit:@cday
Thanks @cday, I installed ghostscript and XnConvert seemed to work fine!
Side note:
The need to install Ghostscript partially defeats the ability to run XnConvert as a portable app (like I do)
I appreciate the info anyway though! and appreciate that sometimes there are practical limitations on things
The DPI setting didn't seem to make much difference for me TBH, it exported OK both with it set and with it not set
I did actually find the "Set DPI" action a bit confusing... I think the control is actually mis-labelled (I think it should say X% and Y%)
... I scanned the PDF at 600 DPI, XnConvert asks for a DPI in "X" and "Y" (600 = approx. 25 x 25) but when I entered this, the output seemed to shrink
... I entered 3000 like you said (3000 = approx. 55 x 55) but this roughly halved the image size
... I entered X = 100 and Y = 100, the image stayed the same size... I entered X = 200 and Y = 200, the image doubled in size!
Anyway, thanks again for your help and am happy to leave the DPI thing for now as it is minor - hopefully someone on the XnConvert team will notice this and, if I'm right, this can be tweaked as part of the next version released.
When Ghostscript is installed the DPI value at which PDF files are rasterised is set to 72DPI. That value is too low to render most images at good quality, so the value used usually needs to be increased. Setting 300DPI often produces a satisfactory quality image, but higher or lower values may be used as appropriate. The filesize of saved image file increases as the value is increased.
In XnConvert the DPI setting can be found in Settings > Load format settings... > Read tab -- PS/PDF -- Resolution.
In XnConvert as far as I'm aware the same DPI value is always used for the X and Y axes. XnView MP, however, does support changing the value used for each axis after a file has been opened, although there is only a reason for doing so in rare special cases.
This is fantastic. (Thanks cday for reminder and link to wiki.) Instructions and Ghostscript install worked great. (I wanted to quickly convert a bunch of PDF artwork that I don't need at massive resolutions or file sizes: I'm using XnView MP to batch convert, PDFs at 300 DPI, resize max side to 7680 pixels, export to JXL at 96+ quality.) One more thing for XnView, perhaps add to wiki: Make sure browser "Filter" (gold funnel icon) on toolbar is also set properly such as "All" or "Custom".