Report development

Hello everyone,
I am currently developing a report allowing the selection of the client as well as the desired month and it displays all the projects assigned to this selected client.
my problem being that when I want to develop a pdf, there is a very long loading time then an error code only with clients with a lot of projects.
also, it is impossible for me to output a usable csv file, when importing csv, I end up with a .csv but with html in it.
If anyone knows how to help me that would be great.
thanks in advance