I saw some ads were scanned as PDF file and posted on the website. Can you explain under what circumstance it is necessary to scan the document into the PDF file?

PDF stands for Portable Document Format. It's a way of presenting a document in a form that anyone can view. Its become the standard format so is used by many.

But its not "necessary" to use the format. Depending on the nature of the document, it could be stored as a JPG, TIF or other formats. But PDF is the most portable and most common.

Is the PDF file size (after saving) less than the JPG, TIF or uploading PDF file is faster than the other file formats, by the way?

It depends on what graphics are in the original format before it was transformed to PDF (or created originally in Acrobat). Generally speaking PDF are larger in size than Word docs or jpgs, but less than tif since if would end up compressing the tif thus losing the original purpose of using a tif file.

Size can be a factor, but compatibility is a bigger one. Every browser can open a PDF. PDF readers are free. There are also many free PDF writers so people can create PDFs.

I saw some ads online are from scanned PDF documents. That means the document was scanned first and then uploaded to the website. It looks like a photo document. Thus, I am unable to edit it. Is it possible to convert it into an editable format (because I need to copy or modify some contents) after I download and save the scanned PDF document from the website?

I am also interested in it - how to edit a scanned document? What is the OCR software?

Yes it is, there are different software available just for this purpose.
I have used Microsoft Office Document Imaging. First I had to convert the pdf into .TIFF image (you can find free online pdf to tiff converters). Then use MS Office Document Imaging, which will let you get the content to MS Word. Then you can edit it.
To answer your question, it is possible, yes.

Optical Character Recognition or Reader software, takes a scanned image and tries to "read" it and covert it into editable text. OmniPro is probably the most popular software in this category. OCR can be up to 95% accurate, but the accuracy goes down if the image is heavily formatted with fancy fonts, etc.

Generally, you read a scanned image into an OCR program and it spits out a Word or Text file with its read of the text in the image.

Can you or anyone recommend reliable free OCR software? I want to try edit a PDF image file. Thanks.