Since the NLM has several resources with almost similar names, it may sometimes be confusing. Some even look much similar as in case of PubMed Central and PubMed. But there are obvious differences between both which can be understood only if you are acquainted with MEDLINE.
MEDLINE in fact is the database of abstracts and citations to biomedical literature that are chosen by a committee constituting a team of expert people in the concerned niche. It includes articles in nearly 5000 journals. The primary purpose of MEDLINE is to intimate us what all are there in the field of biomedicine and italso helps us to locate it.
Most of the researchers and scientists consider PubMed conversion services from expert service providers as a real boon. It is an interface used for searching MEDLINE. It actually serves as an open retrieval system for exploring MEDLINE.
PMC on the other hand is a library. The main aim it has is the collection and preservation of complete text related to life science and not mere references to them. It may find relevant link towards the complete text as it will not be having the text in itself. PMC has full text and it is freely accessible as well but it has very lesser collection when compared with PubMed. The former has 2 million articles and the latter has more than 21 million citations.
In PMC, full text of the articles can be searched and this is not the case with PubMed where only text in its record can be searched. As PMC has digitized some relevant older articles with the help of professional companies offering XML PubMed file conversion services, it would be a better option if you are in need of old information.
A lot of service providers and there for these conversions. Select the best one to assure results that meet your expectations.