Set the size to 100% if you want to print the document. The HP drivers offer a 'fit to page' option, which reduces the A4 in size so that it fits into the printer margins (defined by the printer), that means the printed document will always be a tad smaller than the original.
On the other hand I don't know how Windows handles the PDF files - on the Mac you have a built-in PDF viewer which prints without any flaws.
I don't know if you've already purchased Adobe PDF. There is an alternative: GhostScript, GS View (for Windows only), and a version of Gutenprint. Just check if your HP printer is supported, and you can print everything in PostScript - the GhostScript engine will 'translate' the code to PCL (printer command language) in your printer. GhostScript and Gutenprint are free, GS View costs a few Euro/Dollars.
With GhostScript you can even print directly from Scribus, print the document, print separations, etc.
If you want even more features, you might check 'printfab' for Windows. It's a complete Postscript RIP (Raster Image Processor), also based on GhostScript without any limitations and without the installation hassle. Just make sure that your printer model is supported.