You're trying to combine an A4 contract with a Letter-size invoice and a couple of receipt-size PDFs. Merge them straight and you've got pages of wildly different sizes flowing through one document.
It's not broken — it's just ugly. Here's how to merge mixed sizes properly.
What's actually going wrong
PDFs preserve each page's original size. A merge tool just concatenates pages — A4 pages stay A4, Letter pages stay Letter. Side-by-side they look fine, but in one document the transitions feel disjointed.
For most uses this is fine. For printing, mixed sizes cause problems (printer changes settings mid-job or stops). For visual consistency in a single bundle, you want all pages the same size.
The quick fix
Merge in Flint. If you just need them in one file, the merge works regardless of mixed sizes — pages flow through in order, each at its native size.
If you need consistent sizing, normalise first. Convert each PDF to JPG, then convert image to PDF with a unified page size. The output PDFs are all the same size and merge cleanly.
If that didn't work
For printing, set the printer's page-size handling to 'Fit to page' or 'Auto-rotate and centre'. The printer scales each page to fit the loaded paper, regardless of source size. Output is uniform sheets even though the source document has mixed sizes.
For presentations and distributions where visual flow matters, the JPG-and-back round trip is the cleanest path to uniformity.
Prevent it next time
Standardise page sizes at source. Use A4 or Letter consistently for documents that will be combined. When receiving PDFs in odd sizes (receipts, screenshots, mobile exports), normalise them on arrival rather than every time you bundle them.
FAQ
Will merging different-size PDFs cause errors?
No. Merge tools (including Flint) handle mixed sizes without error. The result is a single PDF with pages at their original sizes. Visual consistency is a different question.
How do I make all pages the same size after merging?
Convert the merged PDF to JPGs, then back to PDF specifying a uniform page size. Or convert each source to a standard size first, then merge.
Why does my printer skip pages when printing mixed-size PDFs?
Some printers can't change paper size mid-job and pause or skip pages that don't match the loaded paper. Set 'Fit to page' in the print dialog and the printer scales each page to whatever's loaded.
Can I rotate pages while merging?
Flint's merge tool keeps pages at their source orientation. To rotate, use the rotate-pages tool either before or after merging.
Mixed-size merging works. For uniform output, normalise first then merge in Flint. Both routes get you to one clean document.