What happens when you merge PDFs
A PDF is a structured container of pages, fonts, images and other objects. Merging copies each page, along with the resources it depends on, from the source documents into a brand-new document. Because the pages are copied rather than re-rendered, text stays selectable and vector graphics stay sharp.
Some document-level features are not carried over by any page-copy merger: interactive form field logic can be flattened or lost, bookmarks and outlines from source files are not combined, and digital signatures become invalid because the document has changed. For everyday documents such as reports, scans and statements, the merged result looks identical to the originals.
Encrypted and protected files
PDFs protected by a password, or encrypted with owner restrictions, cannot be reliably copied. The merger detects encryption and asks you to remove the protection in the original application first. This avoids producing a file with blank or corrupted pages. Very large files, hundreds of megabytes in total, may be slow on low-memory phones; merging on a desktop browser is faster for those.