Why merge at all
A job application consists of a cover letter, a CV and three certificates. A tax return of a dozen receipts. Send that as five or twelve attachments and you make the recipient's job harder and risk something being overlooked. Many portals accept only one file anyway.
The sorting trap
The most common mistake happens before the merging starts. You select every file in the folder, hand them over in one go and trust the order to be right. It often is not.
The reason: file managers sort alphabetically, and alphabetically page10 comes before page2. With twelve scanned sheets the document ends up in an order nobody wanted.
So look at the preview. In Merge PDF you can drag the files into the right sequence before saving. What you see there is the order in the finished document.
What survives the merge and what does not
Filled form fields can lose their function. The visible content remains, but the fields themselves are often no longer editable afterwards. With a completed application form that is usually welcome.
Different page sizes stay different. Mix A4 and A5 and you get a document with changing formats. Technically fine, but printed it looks restless.
Password-protected files cannot simply be merged. The protection has to go first, which is what Unlock PDF is for, provided you know the password.
After merging
Once everything sits in one file, small things tend to surface. A page is sideways, so Rotate PDF. A blank page slipped in, so Delete pages. And if the order still needs changing, Organize pages rearranges it afterwards without starting over.
If the result is too heavy for email, Compress PDF brings it down.
In short
Check the order before saving, because alphabetical is rarely correct. Form fields, mixed formats and password protection are the three places where it snags. Everything runs in the browser, no upload involved.