AI, which isn't always 100% accurate, offers this explanation...
"What you are seeing is confusing but is probably expected behavior from how PicoPDF defines “Fit to Paper Size,” not a literal bug in the sense of wrong math.
From the PicoPDF help:
Paper Size = the physical paper in the printer (e.g., Letter 8.5"×11").
Page Resize: Fit to Paper Size = scale the PDF page so that its full contents fit inside the printable area (accounting for margins, unprintable borders, etc.).
Page Resize: Original Page Size = send the PDF at its original size and aspect ratio; if it is larger than the paper or printable area, it may be cropped.
For many printers, the “printable area” of Letter paper is smaller than 8.5"×11" (for example, a 0.25–0.5 inch unprintable margin on each edge). When you choose Fit to Paper Size, PicoPDF scales the entire PDF page down so that everything fits inside that smaller rectangle, which makes the content visibly smaller than 1:1, sometimes close to the 2/3 effect you’re describing. When you choose Original Page Size, PicoPDF sends the page at 100% and lets the printer handle any clipping or margin enforcement, so it appears “normal” to you.
Why the system dialog prints “correctly”: the OS print dialog you are using is probably defaulting to no additional scaling or “actual size,” and it may be treating the document and paper as the same size (Letter on Letter), so it does not apply that extra reduction step.
So: Your expectation that “Letter + Fit to Paper Size” should be identical to “Original Page Size” when the PDF is already Letter-sized is reasonable from a user perspective, but PicoPDF appears to treat “Fit” as “shrink (or enlarge) so everything fits in the printer’s printable area,” not “only scale if sizes differ.”
That’s why you get different results, and why “Original Page Size” gives you the 1:1 print you want for IRS forms.
Practical workaround and suggestion:
For IRS and other forms already designed for Letter, always use Page Resize: Original Page Size with Paper Size = Letter when printing from PicoPDF.
If you want to push this as a bug or at least a usability issue, you can contact NCH’s PicoPDF support and suggest either:
Changing the default Page Resize to “Original Page Size” when the document and paper sizes match, or
Clarifying the wording of “Fit to Paper Size” to indicate it always scales to the printer’s printable area, which may reduce size even when document and paper are both Letter"