Your A4 PDF is going to a US client whose printer is set up for Letter paper. Without conversion, half their prints will be mis-sized — either with white margins or cropped content.
Resizing to Letter fixes it. Flint's edit tool does it.
Letter dimensions
US Letter is 8.5×11 inches (216×279mm). A4 is 8.27×11.69 inches (210×297mm). Letter is wider but shorter. Direct swap means either some content stretches/scales or some gets cropped. Pick which trade-off you're making before resizing.
Scale to fit
Open edit PDF, choose 'resize pages', target 'US Letter', mode 'scale to fit'. Every page is scaled to fit within the new dimensions. A4 content slightly shrinks because Letter is shorter. Margins adjust. No content is cropped.
Fit and crop
Alternatively choose 'fit and crop' which scales to fill the new size and crops excess. Letter is wider than A4, so scaling-to-fill makes content slightly larger and crops the top/bottom edges. Pick this when whitespace margins are unacceptable but minor cropping is OK.
Preview before saving
Both modes have trade-offs. Preview a sample page before applying to all pages. Scale-to-fit can make small text harder to read; fit-and-crop can lose footers or headers. Try one, check, decide.
FAQ
Will text reflow to fit Letter?
No. PDF doesn't reflow. Pages are scaled/cropped, but text positions are fixed.
Should I just print A4 onto Letter paper?
Most modern printers auto-scale (with margins). Resizing the PDF gives you control over the result.
Does this affect image quality?
Scaling is mildly lossy on raster images. Vectors stay perfect. For mostly-text documents quality is unchanged.
Reverse the conversion (Letter to A4)?
Same tool, different target size.
Pick scale or crop, apply, save. Resize to Letter.