A4 and US Letter look identical until they don't. A4 is 210 × 297 mm; Letter is 8.5 × 11 inches — roughly 216 × 279 mm. Letter is a bit wider and noticeably shorter. Side by side on a desk you'd struggle to tell them apart, which is exactly why they cause so much trouble: the mismatch is invisible right up until you print and find your margins wrong, your footer clipped, or a portal rejecting the file outright.
If someone has told you "this needs to be A4" — or you've received an A4 document and your printer only has Letter — this guide covers forcing every page to the size you need.
When you actually need to do this
- A print shop or office printer loaded with one paper size, and a document built for the other. The printer will scale it, usually badly: uneven margins, shifted content, an unexpected border.
- A submission portal — court e-filing systems, government forms, and regulatory uploaders frequently require a specific page size and check it. They read the page dimensions and reject anything outside spec.
- A document assembled from mixed sources — someone in the US sent Letter pages, someone in Europe sent A4, and now the merged file alternates between the two.
- A template mismatch — a document built for Letter that now has to be filed as A4, or vice versa.
The steps
Open Blackpdf's Resize PDF Pages tool and drop your file in. It reads the document and reports what page sizes it actually contains.
Set the Target size to the one you need:
- A4 — 210 × 297 mm (the global standard)
- Letter — 8.5 × 11 in (North America)
- Legal — 8.5 × 14 in
- Custom… — enter an exact width and height in mm, in, or pt if you need something non-standard.
Set "Apply to" → All pages (Force everything). This is the important one. You want every page to conform to the target — not just the pages that differ from the document's own majority. Forcing everything is what guarantees a uniformly A4 (or Letter) document.
Click Normalize pages and download.
What happens to the content
Resizing changes the page's dimensions, so the content that was laid out for the old size has to be fitted into the new one. The safe default is to fit with padding — the content keeps its proportions and any leftover space becomes margin. Nothing is stretched, nothing is cut.
That's why an A4 → Letter conversion typically leaves a slightly wider margin at the sides and less at the top and bottom: Letter is wider and shorter, so a proportionally-scaled A4 page doesn't fill it exactly. That's correct behaviour, not a bug — the alternative (stretching to fill) would distort your content.
Common questions
Will converting A4 to Letter distort my document?
Not if the content is scaled proportionally, which is the sensible default. Your text and images keep their aspect ratio; the difference in page shape shows up as slightly different margins. Only a "stretch to fill" mode would distort things, and you rarely want that.
The portal still rejects my file even though I set A4.
Two things to check. First, make sure you used All pages / Force everything — if you only normalised the outliers, pages that were already uniform (but at the wrong size) may have been left alone. Second, some portals also check page count or file size, not just dimensions.
Is "A4" actually stored in the file?
No — and this catches people out. A PDF page records a rectangle, not a name. A4 is 595 × 842 points; software recognises those dimensions as A4. Which is why a scan that's a millimetre off isn't "A4" to a strict checker even though it looks identical. Our page sizes explainer covers this in depth.
Should I use Legal?
Only if you've specifically been asked for it. Legal (8.5 × 14 in) is a North American size used mainly for contracts and legal filings. If nobody named it, you want A4 or Letter.
My pages are a mix of portrait and landscape.
Forcing everything to a portrait target will scale the landscape pages down to fit inside it, which may make wide content small. If a landscape page is meant to be landscape (a fold-out table), consider leaving it alone and normalising only the rest.
Can I resize a password-protected PDF?
Not directly. Unlock it first, resize, then re-protect it if needed.
Wrap-up
- Drop your PDF into Resize PDF Pages.
- Set Target size to A4 (or Letter / Legal / Custom).
- Set Apply to → All pages so everything conforms, not just the outliers.
- Click Normalize pages and download.
The key setting is All pages — that's what guarantees a genuinely uniform A4 document. If instead your goal is just to bring a few stray pages into line with the rest, making all pages the same size covers that case.