Start with the output, not the file picker
A reliable merge begins by deciding what the recipient needs. If the result must remain editable, combine files of one compatible office format and expect to review styles, references, and layout. If the result is primarily for reading, printing, or archiving, a PDF usually gives a more predictable final order across mixed source formats.
Make a copy of every source file before merging. Give the copies short, ordered names such as 01-cover, 02-report, and 03-appendix. Open protected or damaged files in their original application first, and remove passwords only when you have permission. These steps make browser processing easier to diagnose and prevent an accidental overwrite from becoming the only copy.
What private browser processing changes
Gluecut reads selected document contents with code running in the browser rather than sending those contents to a document-conversion server. That reduces exposure of the source material, but it does not make every browser or network signal disappear. The website host, analytics provider, and advertising provider can still receive ordinary web-request or interaction data as described in the Privacy Policy; selected document contents are excluded from those systems.
After downloading, inspect every page or sheet that matters. Pay special attention to fonts, page breaks, image cropping, formulas, links, tracked changes, and accessibility tags. Browser libraries cannot reproduce every feature of the desktop applications that created complex files. For signed, regulated, legally filed, macro-enabled, or accessibility-critical documents, use the originating application and your organization's approved review process.