Modifier un PDF gratuitement et sans risque

Hello,

Since you haven't specified what types of modifications you want to make, it won't be very simple.

The software you may be required to use is safe. If you download it from the publisher's website, you won't be taking any risks.

All the software I mention is safe and free.
An important point: only work on copies and preserve the original document.

Several types of modifications are possible:

1) Writing comments and annotations on the PDF. You can use Foxit Reader.
https://www.foxit.com/fr/pdf-reader/

A tutorial here: https://forums.commentcamarche.net/forum/affich-37585770-ecrire-sur-un-document-pdf-avec-foxit-reader

2) Cutting the file or merging several files together. Use PDFSam Basic:
https://pdfsam.org/fr/

3) If you want to modify the content of the PDF, it's much more complicated. Generally, the software isn't free.

There is still a solution. The Draw module of LibreOffice can open a PDF.
https://fr.libreoffice.org/download/telecharger-libreoffice/

You can delete parts of the text, but be careful as you will modify the structure of the document which may become unreadable. This is to be reserved for small modifications.
Once the modifications are made, the file must be exported as a PDF.

What is well conceived is clearly stated,
And the words to express it come easily.
(Boileau)

Hello,

It's obviously impossible without software.

Without getting into sophisticated and paid PDF editors, a number of modifications are possible with, for example, Foxit Reader, provided that the PDF is not secured, and these are essentially annotations, not modifications or deletions of images.

An alternative is to open the PDF in Libre Office, convert it to a Libre Office format, make the changes, and then export it back to PDF.