When I got Adobe Acrobat Pro DC installed on my windows machine it added an add-on function "Save As Adobe PDF" to all of my Microsoft Office applications. This was actually a really cool thing because it allows me to save a document as a pdf to the document's source folder in like a couple of clicks. The only problem is that when I use this function in excel it defaults the page size to something really odd like 15.18 x 19.64 or 12.64 x 9.77 even when I specify "Fit Worksheet to a single page and the page is specified as letter. I think that it is just printing the spreadsheet at "actual size" and does not pay attention to the conversion options.
As per the issue description mentioned above, you want to change the default page size when saving excel file as pdf, is that correct?
Please try following troubleshooting steps:
Launch Excel > click File > Print > select Adobe PDF as printer > printer properties > Adobe PDF Page size. You may refer to this help document for the same: Creating a PDF at the Right Output Size and Dimensions
Go to Acrobat tab > click preferences > and uncheck these option fit worksheet to a single page and fit to paper width > click OK and check.
Let us know if you need any help.