What is kept, and what is not, when you merge
Because pages are copied rather than re-drawn, the visible content of every page comes through unchanged: text, vector graphics, images, page size, rotation and clickable links on the page.
Some things belong to the whole document rather than to a page, and those are not carried over:
- Bookmarks (the outline in the sidebar) are dropped.
- Form fields may still show their values but may no longer be fillable. Fill and save forms before merging.
- Digital signatures stop being valid, because the merged file is a new document.
- Title and author are replaced with neutral values.
For most uses, such as uploading a single application file or sending one attachment, none of this matters. If you rely on bookmarks to move around a long manual, or on a form that others still have to fill in, keep the original files as well and send the merged copy only to people who need one document.
Why the merged file is about the size of the parts
A merged PDF is usually close to the sum of the input sizes, sometimes slightly smaller because the output uses compact object streams. Nothing is recompressed, so a merge of three 2 MB scans will still be around 6 MB.
If a portal sets an upload limit, merge first and then shrink the result with Compress PDF. Do it in that order: compressing each part separately and merging later gives the same total with more steps. When the files are photos rather than PDFs, use JPG to PDF to build the document in one go.
Scans are usually the heavy part
Text-based PDFs exported from a word processor are often only a few hundred kilobytes, while a scanned page can be one or two megabytes on its own. If the merged file is much bigger than expected, look at the size shown on each row: the largest file is the one worth compressing or rescanning at a lower resolution.
Tips for a clean result
Name scanned files with leading zeros, such as 01-cover.pdf and 02-letter.pdf, so that Sort A–Z puts them in the right order straight away. Check the page count on each row: a file showing more pages than you expected may contain blank pages you can remove afterwards with Split PDF.
If a file is rejected as password-protected, open it in your PDF reader, enter the password and save or print a copy without protection. A file reported as damaged often opens again after you re-download it or re-export it from the program that made it.
For applications, a common order is cover letter, CV or résumé, then certificates and references, with the newest first. If the portal gives its own list of required documents, follow that order exactly so the reviewer finds each item where they expect it.
Finally, open the merged file before you send it. Scroll through quickly to confirm that the order is right and that no page is missing.