We had some discussion about the topic here. It covers at least 4 and 5.

UPDATE:

1) Currently it's other way around: if we prefer to store files in database, then we have to configure it to be this way. by default files are stored in filestore, we do not have to configure anything. Files are stored in filestore and naturally they appear there. Thus, database records ("ir.attachment") are necessary as they are pointing to actual files in the filestore. 

2) In this place we see ir.attachment records, any of them may have actual file saved in filestore or in database. It does not matter where file is stored, in database or in filestore, here we can see all ir.attachment records.

3) this setting determine where files are actually stored. value 'file' is default and it means that files will be stored in the filestore. value 'db' means that files will be stored in the database, you can set it to 'db'. also possible use some path, I tried to use path in v7 but it become relative path under filestore... for path you can check how it works for v8. also you can implement support for other storage engines, refer to ir_attachment code. Note: if we change this setting, it'll affect only new records. i.e. if it was set to 'file' and we changed to 'db', then old files will stay where they are in filestore, but new files will be stored in database, and so on.

4) if you interested to do it yourself, then this is completely cowered in the thread that I've referred in the beginning. i.e. create file (csv) and save it using ir.attachment. only difference is that in your case file type is pdf, this difference does not make big deal. But if you just worry that now, it doesn't work... then I can assure you that it works like a charm.

5) Also cowered in the referred thread. files on the disk are NOT saved with it's original names, but using hash code of it's content (used sha1 of content, part of hash used for parent directory of actual file). that's why you can't find the file you search with it's name, but files are there. file names are stored in the database, in ir.attachment. if it's absolutely necessary you can find file on the disk if you know or can compute it's sha1 hash code. you'll find the file under filestore, under directory with a name matching the start (left part) of hashcode. as you find the correct one and you know it's original name, you can rename it (or just add .pdf extension) and open with some pdfreader.