Have you tried adding a Print Setup script step before the Save as PDF script step. That should allow you to change the page size to letter.
The paper size for a PDF is whatever has been set in File > Page Setup or its scripted equivalent. As Todd suggests, to guarantee a specific page size in your PDF output, script that page setup before saving the PDF.
As for your other question, as far as I can see 10.63 x 13.75 inches is not a standard paper size (nearest are US Std Fanfold, 11x14.875, or 11R photographic, 11x14) so I can only assume the ratio of 1.25 has something to do with this size appearing on your system. A maths gremlin, perhaps?
I am mystified. I created a database from scratch, did File > Print Setup ( I don't have Page Setup, running FMPA 13 on Windows 7), and the created PDF is the correct size 8.5 x 11 .
The other database I am having issues with was created in FMPA 12 on a Mac. The guy told me the only to have reports correctly fill a Letter sheet (portrait) was to make it 13.75" wide and then to print the PDF a scaled to fit.
Looks at the origin something was kind of wrong, but what ?!?
Sometimes, it remembers that last page size that was printed. If you choose Print Setup before saving as PDF it should clear that out. I Googled that paper size. Oddly ( and probably irrelevantly) that is the size of a standard serving tray like at a fast food restaurant. Maybe someone printed out a placemat.
I wonder if "the guy" was trying to adjust for printing a layout which hadn't been optimized for print, and that was the only way he could think of make it fit.
Of course, any PDF which is printed with the Scale to fit setting will fit whatever current paper size is set for the printer when printed from Acrobat and other PDF programs which support that setting.
Are you using only the pdf print resident in Filemaker? Do you also have a full version of adobe available to print from? If so have you tried to use it as a printer and get the same result?
In my case when a pdf is needed I use the full version as I also had a similar issue. Moving forward my reasons have changed as I like a little more control of the pdf creation. I also had some strange file sizes when comparing them to the Filemaker PDF generator.
The Filemaker version seemed a little to stripped for my purposes.