PMC documents should be downloaded from the URL provided under `link`

[lfoppiano]

While downloading PMC documents sometimes the retrieval of the PDF fails because there is no unpaywall link in the glutton response.

For PMC the url could be found under link:

Example of PMC ids:

1325037
2096715
2254971
2276181
2633482
2652022
3033436
3404965
3716414
3955154
3998744
4074849
4246549
4261204
4430278
4988499
5096026
5127879
5525363