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Italian Citizens do not need a visa to visit Canada for short periods, provided they satisfy the other restrictions (mainly the 'not working in Canada' ones).
The supporting documents you need are essentially ones that show the immigration officer that you will not violate the terms of visiting. I would recommend a letter from your company stating the purpose of your visit, and possibly a similar letter from the customer, even though you don't need a visa. You may find you don't need it. I have entered Canada as a British Citizen many times on business (as a Software Engineer for meetings with a customer) and not been asked to show any supporting documents. But better safe than sorry.
Canadian Visitor Visas come in either Single Entry or Multiple Entry. With a multiple entry visa you can enter Canada as many times as you like. With a single entry visa you can enter only once (although you can visit the US or St Pierre and Miquelon and return to Canada).
As of February 2014 Canada will generally issue multiple entry visas, if you are eligible, even if you did not ask for one. However if you need one you should still ask for one.
Upvote:7
I recently traveled to Canada for business. At the Immigration I presented my Invitation Letter (basically a document signed by company that asked me to come to Canada with all information about my work and the reason for which they need me in Canada). This document and the return ticket have been enough to enter in Canada for business.
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I'm a freelance software engineer based in Italy. The letter must contains all personal details and the details of one contact of the company (who signed the letter). This is the main part of the letter: