In XDPXv16.3 for some obscure (Magix) reason when you select Help > PDF Manual you are expected to download the same file for every access.
Help > PDF Manual launches https://rdir.magix.net/?page=APQN0PQHQ3FD, which in turn goes to http://dl03.magix.net/manual_designerprox16_en.pdf.

This is the workaround I use.

Do the above for the first time (and only) and rename the PDF file to Manual - 16dot3.pdf.
Close XDPX - you don't have to make it maintains the voodoo.

Find you version of this folder: file:///C:/Program Files/Xara/Xara Designer Pro X/16/HelpAndSupport/ENG/.
It should contain xarax.chm and possibly earlier versions of the PDF Manual; these started from v16.0.

Locate C:\Program Files\Xara\Xara Designer Pro X\16\Product_eng.cfg and use a text editor with admin privilege, change this line:
Code:
PDFManualLink=https://rdir.magix.net/?page=APQN0PQHQ3FD
to
Code:
PDFManualLink=file:///C:/Program Files/Xara/Xara Designer Pro X/16/HelpAndSupport/ENG/Manual - 16dot3.pdf
If worried, copy the file first.

Now Help > PDF Manual opens your locally-stored version.

Acorn