How to see xml coding of SVG image created in inkscape

This functionality has been implemented in Inkscape 1.0:

• right-click on object
• select "Create link"
• enter target address under "Href"
• store as PDF using "Save a copy..."
• (I also restricted the output to PDF 1.4)

My system: Linux Mint 19.3, Inkscape 1.0+r73+1.

As you are using vector graphics, the resolution doesn't really matter. It will matter if you export it to a bitmap or if you print it (as a bitmap, but you can also print as a vector in which case the resolution doesn't matter), at which times you will have to set a proper resolution.

So you don't have to set the resolution on your image, if you export to bitmap in a different format than SVG you will have a field where you can set your resolution on the export dialog. If you print, you'll have to set the resolution on your printer (you'll probably find some option for it in the print dialog). If you are using bitmaps in your SVG file, make sure they are at the proper resolution before importing them into Inkscape.¹

This also can be done by creating a new SVG image. Since SVG images and their behaviors are defined in XML text files, creating a new SVG image will eliminate the low resolution artifacts that were observed in your previous 25x25 millimeter SVG image.

¹_{Rico's answer to How do I set resolution in Inkscape?}