enclosed letter

Hi, could you please look at the following enclosed letter? Is it commonly acceptable to start almost each sentence with the pronoun? Is the word “collection” correctly used here?

Dear Ms. ….,

We are pleased to send you two copies of “Electronic Libraries’2017” Collection with articles by your colleagues:
Web Archiving for Newspaper Preservation by Dr. Björn Nordqvist (pp. 118 – 130);
Legal Deposit for Online Materials and Its Perspectives by Ulf Tegnér (pp.129 – 132).
We would like to thank you for the provided materials on best practices of the National Library of Sweden. Undoubtedly, this experience will be of interest to LIS professionals. We look forward to continuing cooperation.

Kind regards,

Well, as there are only four sentences in the letter, I don’t think it matters that three of them start with the same pronoun. However, your question should be ‘almost every sentence’, not ‘almost each sentence’.

You need the definite article:
‘… two copies of the “Electronic Libraries’2017” Collection …’

Collection’ is correct, but ‘experience’ isn’t correct with ‘this’. It needs a pronoun.
your experience…
their experience…

If you want to avoid using the pronoun twice, one of severla possible resolutions would be this (I’ve ‘neatened up’ several other things too):
Please find attached/enclosed two copies of the Electronic Libraries 2017 Collection containing articles by your colleagues:

enclosed - for standard mail attached - for email

Thank you for providing material on best practices at the National Library of Sweden. Undoubtedly, this information (or your experience, as indicated above) will be of interest to LIS professionals. We look forward to your continuing cooperation.

Thank you! Is it always necessary to put the definite article before the names of the scientific publications? Does it depend on the type e.g. collection or journal?

It depends on the sentence rather than the type of publication. Generally, nouns need an article.