I was wondering if it's possible to édit an existing pdf file with Pdfminer. It seens to be a powerful tool, but the documentation is poor/inexisting.
I found some exemples, but they don't match with my goal. I want to make a search engine which changes the color of my keywords in the pdf file.
PDFMiner is not for altering existing PDF files, but for extracting text and metadata from them.  The closest solution to what you're looking for using PDFMiner would probably be to use the included pdf2txt.py tool to extract the text and then mark that up to highlight your keywords.
There's also the simple option of just using a PDF viewer with the built-in ability to find and highlight multiple search terms.  I think Adobe Acrobat can do it, but I'm not sure about others.
Edit pdf file with PDFMiner