I'm sharing only my limited experience and humble knowledge. If you dig more through this forum you may find plethora of cases.
What I did, based on some previous immigration experience and suggestions of my lawyer:
1) if you have more than a single document supplied per section (e.g. 2 passports, proof of work related things comprised of multiple items, police clearance etc), you rather compile a pdf document with Table Of Contents listing all the documents included.
In my first rejection one of the reasons was allegedly missing document, which was in fact there.
2) "Трудовая книжка" is loved by CIC for proving your work experience, if you have one.
3) I added Letter of Explanation adding comments to all my work experience, especially for those jobs that were done as a Private Entrepreneur. I had references from my employers but still felt it was better to explain some details which could seem dubious.
4) Beware, that max size of the PDF you can upload per section is 3.5Mb so you need to use some kind of PDF compressing tool. I used this one pdfcompressor.com but there're others available as well.