How do I print free PDF plans at the right size when my printer wants to shrink them?

Print shop is the easy answer, but home printing works fine once you know the trick. For anything bigger than A4 or letter I take the file to a copy shop and ask for an A3 or A1 print. Costs a couple of quid and you get the whole plan on one sheet with no tiling headaches. But for a micro foamie the pages you've got are fine. Yes, 'actual size' is exactly what you want, and four pages chopped up is normal because the PDF was probly drawn at full size across multiple pages. To tile it back together just trim one edge of each page with a sharp blade and steel ruler, then line them up on a window or lightbox if you have one, or against a bright lamp. Match the lines themselves, not any corner marks, since like you noticed not every plan has registration crosses. A bit of low-tack tape on the back holds it all while you check the alignment. If the plan has a known feature, like a fuselage length or a wingspan dimension written somewhere, measure that across the taped-together sheets to confirm it came out right. One thing worth checking: some printers scale slightly even on 'actual size' if the driver has a 'fit to page' default hiding somewhere. Print a test square or the calibration page if the PDF has one, measure it, and you'll know straight away wheather your printer is honest.

As for the 97 percent question, a 3 percent shrink on a micro depron plane genuinely doesn't matter. Foamies fly on wing area and control throws, and 3 percent of a 600mm wing is 18mm, which you'd compensate for by just moving the battery a few mm. What WILL bite you is mixing scales, so if one part prints at 97 and another at 100, the parts won't mate. Print everythign in the same plan the same way and it's all consistent. Honestly I've built from plans that got shrunk to fit and the only fix needed was a bigger servo hole.

Before all this PDF stuff we used to trace plans out of magazines onto greaseproof paper and cut the foam straight off that, so tiling four pages is luxury as far as I'm concerned. One thing I'd add, after you tape the sheets together, draw a long centreline down the fuselage with a straightedge before you cut anything. If the tiling is off by a couple of mm anywhere, you'll spot it against that line instead of after the glue dries. Saved me more than once on profile builds. I also build the fuse first and dry-fit the wing against it before committing to the tail surfaces, just in case something came out longer than the plan says.

One thing I do differently is skip 'actual size' altogether and use the poster or tiling print mode in the PDF reader instead, because that lets you set an explicit overlap between sheets, say 10mm, and it prints little alignment ticks for you. Trimming pages then becomes optional since you can just fold the overlap back or butt it up and the doubled lines show you exactly where you are. Sounds fiddly but it saves arguing with a lightbox at 11pm. Also worth knowing that some plans include a printed scale bar or a stated dimension somewhere in the title block, and I've had at least one download where the drawing itself was drawn at a diferent scale to the sheet, so checking one known length before you cut anything has saved me a sheet of foam more than once.

One thing I do that saves all the faffing about with tracing: spray glue the printed sheets straight onto the depron and cut through the paper. I use the repositionable stuff so it peels off afterwards without tearing the foam. Cut the fuselage sides this way and they came out dead symmetrical, which tracing never managed for me. Only downside is you burn through blades faster because the paper dulls them quick, so I keep a few spares on the bench before starting a build. Also print two copies of anything with a fold line so you can keep one clean sheet as reference if the glued one gets mangled mid-build.

Update: sorted. Tried the poster print mode in the PDF reader like someone suggested, set the overlap and it put the alignment ticks on for me. Taped the sheets up and measured the test square, spot on at full size. Then sprayed the sheets onto the foam and cut thru the paper, worked a treat. Fuselage sides are symmetrical and I just need to cover the top and bottom now.