no, you can't edit an existing pdf, the nonstandard form filling extension notwithstanding. you can extract as much information as possible from it and recreate it. that's what "pdf editors" are doing. and since it's not officially supported, any edit can screw the file up.

the reason you can't just edit it is that pdf is basically a container for program code that runs on printers. so you can have text interspersed with formatting information, or text with non-existent characters approximated by vector images, or text that's been rendered to a raster image and is not actually in the document. then you have the fact that pdf can embed specialized fonts, compressed files, security measures, and even internal programs. and it's all offset-based in there so you need to modify the entire file structure in order to get it working again after adding text. what's worse, since any file with a pdf document in it is a valid pdf document according to the spec, less reputable "pdf editors" can just embed whatever shit they want. it's a common malware vector.

it's much safer to re-build the document from source. if you don't have the source, there are tools to extract just the textual content.