A form portal rejects your upload because it "must be A4". A US client sends Letter pages and your printer in Europe keeps asking for paper it doesn't have. A publisher wants a 6 x 9 inch PDF and you've got A4.
All of these are the same job: change the size of the page and decide what happens to the content on it. Resize PDF does that in your browser, and the content stays real text and vector graphics, so nothing goes blurry.
Page size and content size are two different things
In a PDF, the page size is a single rectangle stored with each page, measured in points (72 points to the inch). A4 is 595 x 842 points. Letter is 612 x 792. That rectangle is the sheet. The content (text, lines, images) is drawn onto it using its own coordinates.
So changing the page size on its own doesn't make anything bigger or smaller. It just moves the edges of the sheet. Whether the content shrinks, grows or stays put is a separate decision, and that's the one people get wrong.
Resize PDF changes both in place. It sets the new page rectangle, then wraps the existing content in a scale-and-move instruction. Because nothing is redrawn as a picture, text stays selectable and searchable, links still work, and form fields and bookmarks are kept. Comments and links are moved along with the content so they still sit over the right words.
Letter and A4 don't have the same shape
This is why a straight swap never looks quite right. Letter is 8.5 x 11 inches. A4 is 8.27 x 11.69. A4 is narrower and taller.
When you scale Letter content to fit an A4 page, the width is the limit. The content shrinks by about 3% and there's roughly half an inch of extra white space at the top and at the bottom. Going the other way, A4 to Letter, height is the limit. The content shrinks by about 6% and you get a bit over a third of an inch of extra space on the left and right.
Neither is noticeable on a normal document. It does matter if your page has a border or a letterhead that's meant to sit right at the edge, because it won't any more.
Resizing a PDF step by step
- Open Resize PDF and add your file.
- Leave "Change page size" selected and pick a size from "Page size". A4 is the default. The list also has A0 to A6, B4, B5, Letter, Legal, Ledger, Tabloid and Executive, each shown with its size in inches.
- Set "Orientation". "Auto" keeps landscape pages landscape and portrait pages portrait, which is what you want for mixed documents.
- Choose what happens to the content under "Content" (more on this below).
- To resize only some pages, type them in "Pages", for example "1-3" or "odd". Leave it empty for all of them.
- Click "Resize PDF" and download the file. It's saved as resized-yourfile.pdf.
For a size that isn't in the list, choose "Custom size" and type a width and height. You can work in inch, cm, mm or pt, and switching unit converts the numbers you've already typed. Anything from 0.5 in (13 mm) to 200 in (5 m) is accepted on each side.
Scale to fit or keep size
"Scale to fit" makes the content as large as it can be on the new page without distorting it. The proportions are kept and the content is centered, so the leftover space is split evenly between two sides. This is the right choice almost every time.
"Keep size (center)" doesn't scale at all. The content is placed in the middle of the new page at 100%. On a bigger page you get a white border. On a smaller page, whatever doesn't fit is cut off at the edges. I'd only use it when the size of the printed content really matters, like a technical drawing at a fixed scale or a label that has to match a sheet of stickers.
Under "More options" there's "Page margins". This keeps a strip of white space free on each edge of the new page and scales the content to fit inside it. Useful when the original has text running almost to the edge and your printer can't print that close. If you set margins larger than the page can take, you'll see "The margins leave no room for the content on this page size."
Adding margins without changing the content
The second mode, "Add margins", works the other way round. Instead of fitting content into a fixed page, it keeps each page exactly as it is and grows the page by the white space you ask for. Each side defaults to 0.5 inch.
That's what you want for binding (extra space on the left so the hole punch misses the text), for leaving room to write notes, or for a scanned page where the text touches the edge. The content isn't scaled, so text stays the same size. The page just ends up bigger than any standard size, so if it needs to be A4 afterwards, run it through again with "Change page size".
Resizing isn't the same as printing to fit
Most print dialogs have "Fit" or "Shrink oversized pages", and for a one-off printout that's fine. The difference is that the print setting only applies to that one print job on that one computer. Resize PDF changes the file, so the next person who prints it, or the upload portal that checks the page size, sees the new size.
A few things it won't do. It won't reflow text. Making a page narrower makes everything smaller, it doesn't rewrap paragraphs. If the problem is big empty margins rather than the wrong paper, Crop PDF the margins off first and then resize with "Scale to fit", which makes the text noticeably bigger on paper. And if you want two pages on one sheet rather than one page on a different sheet, that's N-up; the guide on printing multiple pages per sheet explains it.
Check a page with a different shape
If your document mixes portrait and landscape pages, or has a fold-out page in a bigger size, open the result and look at those pages specifically. With "Orientation" on "Auto" they should keep their direction, but forcing "Portrait" or "Landscape" turns every page that way and shrinks the odd ones out to fit. When only a few pages are the wrong size, put just those page numbers in "Pages" and leave the rest alone.
