Good question though -- what exactly is IrfanView doing (didnt even know it had that capablity).
If I'm downloading a PDF I will open it directly, rather than save, and then Crtl+Shift+S will allow me to save it with new and/or modified name.
Software tends to remember where you last saved and go directly there, which is mostly convenient. (I use Adobe Reader as PDF viewer.)
Depending on your browser, opening directly can be made the default behaviour (in Pale Moon here, clicking a PDF link will open the Open/Save dialogue with 'Open with' selected by default, I just have to press return).
As I say, a workaround, throwing it out there as it might meet OP requirements.