Why page sizes cause trouble
Documents from the USA use Letter size, while India and most countries use A4. Printing a Letter PDF on A4 paper — or the other way round — cuts off edges or leaves odd margins. Merged PDFs can also mix page sizes, which portals and printers dislike.
How it works
Choose A4, US Letter, US Legal, A3 or A5, and the orientation: keep each page's own orientation, or force portrait or landscape. The content of every page is scaled to fit the new size and centred, so nothing is cut off. The content is embedded as vector graphics, not turned into an image, so text stays sharp and selectable.
What is not carried over
Clickable links, comments and form fields belong to the original page frame and are not carried over to the resized pages. If you need fillable fields, resize first and add fields afterwards with Create Fillable PDF Forms.
Common problems and how to fix them
Margins look larger: the page shape changed, so the content is scaled to fit with space around it. Landscape tables became tiny: choose “Same as each page” to keep them landscape. A rotated page looks wrong: fix rotation first with Rotate PDF.
Good habits
Use one paper size for the whole document, print a test page before printing many copies, and keep the original PDF.