PHP get pdfs and combine

How it's possible to get the pdfs and combine them into one using checkboxes?

For example I have an array of links and I select a few and download only one .pdf (combined).

For example:

<input type="checkbox" name="category1" value="link1.pdf">
<input type="checkbox" name="category1" value="link2.pdf">

<input type="checkbox" name="category2" value="link3.pdf">
<input type="checkbox" name="category2" value="link4.pdf">
<input type="checkbox" name="category2" value="link5.pdf"> (links are not numbered, may vary)

For example I select checkbox link1.pdf, link3 and link5.

I check this answer (without libraries) but it's using .zip and the files have the same name. (I say this because all +100 links are already uploaded into wordpress and I cannot refactor them all)

Is this possible? The category(name param) must be the same? Thank you