1. You have a schematic in PDF format, but it's hard to read when printed on A4
2. Only A4 printer available
3. Printer drivers can't solve the problem
4. A3 size wanted (ie two sheets A4, with some overlap for taping together)
5. Following (free/OSS) programs are available:
To illustrate point 3: my printer will print multiple pages per sheet ("2 in 1" - ie reduce), or print an A4 page on (n x n) sheets (A2 - four times the area), but there's no option to double the size to A3. A4 too small; A2 too big
About GIMP - https://www.gimp.org/
Open source image manipulation program - sort of free Photoshop, plenty of info around.
About PosteRazor - PosteRazor - Make your own poster!
PosteRazor is designed to split image files into [n by m] individual pages in a single PDF file, on whatever page size...