What this tool does
Printing an A4 document on Letter paper, or the reverse, is a small annoyance that produces a specific and recognisable failure: the last line of every page is missing, because a Letter page has an unprintable margin that the A4 layout ran into. Fixing it means changing the page size properly, which is a different operation from simply editing a number.
How it works
There are three ways to change a page size in a PDF and they are not equivalent. Changing the box alone crops whatever no longer fits, so a long line gets cut off at the edge. Changing the box and stretching the content to fill it distorts the page, making circles oval and text unevenly spaced. Changing the box, scaling the content uniformly by the smaller ratio and centring it adds a margin instead. Only the third is right for a document, and it is what this tool does by default, with the other two available when a specific job calls for them.
The numbers are exact rather than tidy, and printers care. A4 is defined as 210 by 297 millimetres, which at 72 points to the inch is 595.276 by 841.890 points. US Letter is 612 by 792 points, which really is round, because it is defined in inches. The consequence is that converting between them always leaves a leftover: an A4 page scaled to fit Letter height comes out about 26 points narrower than the sheet, so the margin lands on the left and right rather than the top and bottom. Getting the direction right matters, and the tool reports which way the space went.
The unprintable margin is why converted documents lose their last line. Most home printers cannot print within about a quarter inch of the edge of the paper, so a document laid out for one paper size and printed on another can push content into the area the printer skips. The tool states this as a known limitation rather than treating it as a conversion error, because no amount of page-size conversion fixes a printer that cannot reach the edge of its own paper.
Documents are frequently mixed, and the tool says so rather than assuming. If a PDF has 40 A4 pages and three landscape pages, it reports the dominant size and how many pages differ, and offers to apply the change to everything or only to the pages matching the dominant size. Converting only some pages of a document is occasionally what you want and is rarely what you meant.
Worked example
An A4 contract has to be printed on Letter paper at a US office, and the last line of each page is being cut off.
- Read the page report: 20 pages, all A4 at 595.276 by 841.890 points
- Choose US Letter and the fit mode, which scales uniformly and centres
- The report shows the leftover went to the left and right, about 26 points per side
- Apply to all pages, or only to A4 pages if the document is mixed
Every page is Letter with the content scaled and centred, with no cropping and no distortion, and you know why a converted file might still lose a line to your printer's unprintable margin.
Accuracy and limitations
- A home printer cannot print to the edge of the paper, usually not within a quarter inch, so a converted document can still lose content on the printer even when the page size is correct.
- Fit mode adds a margin to preserve the aspect ratio. Fill mode clips the overflow, and stretch mode distorts the content; both are available but neither is right for ordinary documents.
- Links and annotations are not scaled with the page content, so elements added by a stamp or a form can end up out of position after a resize.
- Very small or very large page sizes are rejected rather than allowed to produce files readers cannot open.
Frequently asked questions
- How do I convert a PDF from A4 to Letter?
- Choose US Letter as the target and use the fit mode, which scales the content uniformly to the smaller of the two ratios and centres it, adding a margin on the sides rather than cropping or distorting. The report tells you which way the leftover space went.
- Why does my printed PDF lose the last line of each page?
- Because most printers cannot print within roughly a quarter inch of the edge of the paper, and a document laid out for a different paper size can run into that band. This is a printer limit rather than a conversion error, so changing the page size back or choosing a page size with more margin is the fix, not another conversion.
- Should I use fit, fill or stretch?
- Fit for documents, because it scales everything uniformly and centres it, so nothing is cut off and nothing is distorted. Fill when you want the page to be completely covered and accept losing the overflow. Stretch is only correct when the source and target have the same aspect ratio, which is rare, and it distorts the page otherwise.
- What exactly is A4 in points?
- A4 is 210 by 297 millimetres, which is 595.276 by 841.890 points at 72 points to the inch. You will see 595 by 842 quoted often, and that is a rounding of a real measurement rather than the specification. US Letter is 612 by 792 points, which is exact because it is defined in inches.
- Can I convert a PDF to a custom page size?
- Yes, enter your own dimensions in millimetres or points, or pick from the named sizes. Custom sizes are useful for labels, badges and shipping documents where the paper is not a standard size at all.
- Do signatures and stamps move when the page size changes?
- Stamps drawn on top of the page content are scaled with it, but elements that are positioned independently, such as a form field or a link, may end up out of position afterwards. Check the result on a page that has them before converting a whole document.